A bright comet is one of the most spectacular astronomical sights available to the naked eye. Since Comet Hale-Bopp put on a long-running show in 1997, sixteen years have passed without such an impressive display. However, this is about to change.

Comet ISON may give us a once in a lifetime light show, when it probably will become the brightest object in the night sky. How bright it could get is currently the subject of debate, but on this page we will cover Earth’s encounter with the comet in great detail. Stay tuned!

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December 19 – Goodbye ISON!
November 29 – Amazing Comet ISON survives perihelion
November 28 – ISON on perihelion day
November 21 – Expect a spectacular display in early December
Archived Updates


1. First things first: What exactly is a comet?
2. From where does Comet ISON come?
3. Who discovered Comet ISON?
4. Why is the comet named ISON and not Nevski-Novichonok?
5. How close will Comet ISON come to Earth? And to the Sun?
6. I hear that Comet ISON will pass very close to Mars. Is this true?
7. How fast will Comet ISON move across the night sky?
8. What is the size of C/2012 S1 ISON?
9. How bright will Comet ISON be?
10. Is there any link between Comet ISON and the Great Comet of 1680?
11. Could C/2012 S1 ISON create a meteor shower?
12. Will Comet ISON be visible from Australia and the Southern Hemisphere?

Viewing Guide

Viewing and photographing Comet C/2012 S1 ISON
Comet ISON in August 2013
Comet ISON in September 2013
Comet ISON in October 2013
Comet ISON in November 2013
Comet ISON in December 2013
Comet ISON in January 2014


December 19 – Goodbye ISON!

Just hours after our last optimistic updated, Comet ISON started to fade dramatically, at the rate expected of a simple, inactive debris cloud moving farther from the Sun’s illumination. Unfortunately, the renewed and rather substantial coma failed to evolve.

The dust remnant was visible on STEREO images until December 6, and a few observers managed to detect it in backyard telescopes. Estimated magnitudes were +7.5 on December 6, +7.2 on December 7, and +9.6 on December 9. The Hubble Space Telescope failed to detect fragments of ISON on December 18.

November 29 – Amazing Comet ISON survives perihelion

Yesterday, when Comet ISON plunged through the solar atmosphere and behind SOHO’s coronagraph (the black disk designed to block out the direct light from the Sun), its nucleus dwindled away to nothing and most of the tail simply evaporated. Everyone assumed that the comet completely disintegrated and died a fiery death.

However, several hours after perihelion, ISON began to brighten up again. It is now distinctly evident on live images from SOHO, looks like a comet, and continues to brighten as it moves farther away from the Sun. The estimated magnitude is between +0.5 and +1!

Several scenarios might explain ISON’s peculiar behavior:

If the nucleus has broken up into several smaller pieces, than this cloud of mini-nuclei continues to emit dust and creates the dust fan we are seeing now. Rather than a condensed coma, we now see a diffuse extended coma that continues to spread out as individual pieces of the comet move apart. In this scenario, the comet will continue to shine for days of even weeks from now.

The second scenario implies that much of the comet’s coma consisted of small dust particles that were vaporized by the intense heat of the Sun. In this case, ISON should regrow a strong coma and tail as it moves away from the Sun. If we are lucky, enough of the nucleus remains in one piece to reform a more substantial coma and tail.

This happened before, in 1962, with another sungrazer comet. Comet Seki-Lines was bright prior to and after perihelion, but it was not observable at the time of closest approach to the Sun, though it should have been an easy object. It was almost as if the comet turned “off” near perihelion, and then “on” again.

The third scenario – we are seeing something we have never witnessed before. There were predictions of breakup, disintegration, survival, etc. And now? ISON is an extraordinary comet, a dynamic and unpredictable object that continues to amaze.

The big question now. Will ISON be visible with the naked eye in the following days? Unfortunately, there is no answer for that. Any answer would be highly speculative.

Comet ISON After Perihelion
The latest image from SOHO’s wide-angle (C3) coronagraph. Comet ISON is clearly visible above center. NASA/ESA/SOHO Consortium

November 28 – ISON on perihelion day

For those who closely followed news regarding Comet ISON, the last two days have been quite a roller coaster of a ride. Despite observations with the IRAM millimeter telescope in Spain, which indicated that Comet ISON has completely disintegrated, NASA’s Solar and Heliospheric Observatory (SOHO) is just now sending amazing images of a healthy, very much alive comet.

For the moment, no one knows if ISON still has an intact nucleus, or if it will survive its close brush with the Sun today. What we do know is that the comet is still alive and brightening dramatically (at least magnitude -2.5 so far). It is conceivable that ISON will become bright enough to spot in broad daylight today, in the hours immediately before or after perihelion (please see the Viewing Guide for November).

Essential Links:

Latest SOHO Images – near realtime images and movies

SDO Views Comet ISON – near realtime images and movies

NASA’s Comet ISON Observing Campaign – updates

November 21 – Expect a spectacular display in early December

Starting today, Comet ISON is too close to the Sun for any Earth-based observations. After a second smaller outburst on November 19, the latest visual estimates placed the comet at magnitude +3.5 or +4. Until now, there is no hard evidence that ISON fragmented during the two outbursts; if the comet survives perihelion expect a SPECTACULAR display in early December, when ISON returns to the morning sky.

There is a good reason for using all caps for the word “spectacular”. For readers who remember Comet Lovejoy’s beautiful display in December 2011 (visible only from the Southern Hemisphere), ISON is now about one hundred times brighter than Lovejoy was at the same distance from the Sun! Furthermore, ISON is estimated to be ten times bigger than Comet Lovejoy.

Although no longer visible from Earth at the moment, Comet ISON is just entering in the field of view of NASA’s fleet of orbiting observatories. MESSENGER and STEREO-A HI1 will soon begin observations, and in a few days we should have images from both spacecraft.

Comet Lovejoy in December 2011
A sungrazer like ISON, comet C/2011 W3 Lovejoy (pictured above) put on a stunning display for Southern Hemisphere observers in December 2011. ESO/Yuri Beletsky


1. First things first: What exactly is a comet?

Comets, often referred to as “dirty snowballs”, are bodies composed of ice and dust, which measure only a few miles across. We normally think of a comet as having three parts. The head of the comet consists of the coma and the nucleus. Sweeping away from the coma is the comet’s tail, which can often be as long as the distance from the Earth to the Sun (almost one hundred million miles!).

When the comet is away from the Sun, the nucleus is all there is, and the comet is too small and too faint to be seen. As it approaches the Sun, the nucleus becomes warmer and ices inside evaporate, carrying also the dust that was embedded within. This is what makes up the comet’s coma. Because the Sun’s radiation pressure and the solar wind exert a force on the coma, a gaseous tail is formed, pointing directly away from the Sun.

Comet Diagram
Radiation pressure and solar wind effects result in a comet’s tail always pointing directly away from the Sun. NightSkyInfo.com Diagram

2. From where does Comet ISON come?

The comet comes from the Oort Cloud, a swarm of frozen ice and rocks orbiting the Sun at a distance of about one light-year. The cloud may contain up to a trillion comets, with a total mass of more than the mass of Jupiter.

Because at some point another cometary body in this cloud passed near ISON, it caused it to change its path, plunging the comet toward our inner solar system.

Oort Cloud Diagram
Comet ISON comes from the Oort Cloud, a reservoir of comet-like bodies located in the outer fringes of the solar system. NASA Diagram

3. Who discovered Comet ISON?

The comet was discovered on September 21, 2012, via images taken with a 16-inch reflector telescope that is part of the International Scientific Optical Network (ISON). This is a group of observatories specialized in finding asteroids and space debris, with facilities in ten countries.

In the wee morning hours of that day, Vitali Nevski and Artyom Novichonok were recording images in the constellations Gemini and Cancer. They were using the 16-inch reflector at Kislovodsk Observatory in Russia, part of the ISON network. After running the images through CoLiTec, software used to detect asteroids and comets, Nevski spotted the object now known as ISON.

Novichonok explains: “We could not be certain that it was a comet, because the scale of our images is very small, and the object was very compact.” Other observers at the International Astronomical Union’s Minor Planet Center in Cambridge, Massachusetts, soon confirmed the discovery and announced it on September 24.

When first sighted, Comet ISON was 625 million miles (one billion kilometers) from the Earth and glowed at magnitude +18.8. This is a scale used by astronomers to measure the brightness of objects in the sky (the lower the number, the brighter the object). A magnitude of +18.8 means that at the moment of discovery the comet was about 100,000 times fainter than the dimmest star visible with the naked eye!

Comet ISON Discovery Image
The discovery image of Comet ISON, recorded by Vitali Nevski and Artyom Novichonok. Vitali Nevski/Artyom Novichonok/ISON

4. Why is the comet named ISON and not Nevski-Novichonok?

Comets are usually named after their discoverers, but because the initial report from Nevski and Novichonok did not indicate a cometary appearance for this object, and other observers confirmed its cometary nature, the comet was generically named “ISON” (after the telescope network used to discover it).

The comet’s formal designation is C/2012 S1. The letter “C” shows that ISON is a non-periodic comet, 2012 is the year of discovery, and the upper-case “S” indicates the half-month when it was discovered. The “1” after the letter “S” is just a sequential number.

5. How close will Comet ISON come to Earth? And to the Sun?

On closest approach, the comet will pass within 39 million miles (63 million kilometers) from Earth on December 26, 2013. This is 42-percent of the Earth-Sun distance. ISON poses no threat to our planet, and even if the comet breaks up into fragments, these will continue along the same safe trajectory as the original comet. Just sit back, relax, and enjoy the celestial show. Comet ISON does not come remotely close to us!

ISON is a sungrazing comet, meaning that its orbit will bring it very close to the Sun. It will reach perihelion on November 28, 2013, at a distance of only 1.1 million miles (1.8 million kilometers) from the center point of our star. If Comet ISON survives this close encounter with the Sun, it could emerge glowing as brightly as the Moon and easily visible near the Sun in broad daylight.

Comet ISON Orbit Diagram
Comet ISON will pass very close to the Sun in late November 2013. Astronomers say Comet ISON’s orbit is similar to that of another comet that passed by Earth in 1680 and was noted for its spectacularly long tail. You can examine the orbit of Comet ISON with the JAVA orbit tool provided by the STEREO SECCHI team. NASA/JPL Diagram

6. I hear that Comet ISON will pass very close to Mars. Is this true?

Yes, on its way to the Sun, Comet ISON will first pass very close to Mars. On October 1, 2013, ISON will be only 6.7 million miles (10.8 million kilometers) from the Red Planet. At that time, it may be visible to rovers and orbiting spacecraft, becoming the first comet in human history to be observed from more than one planet. Mission controllers at NASA’s JPL may try to observe it with the cameras onboard the Curiosity and Opportunity rovers. We will keep you updated.

View of Comet ISON from Mars
On October 1, 2013, Comet ISON will have a close encounter with the planet Mars. Above, a view of Comet ISON from Mars’ Gale Crater, close to Curiosity’s landing site. The date is October 1, just after the Martian sunset. Image generated using the Starry Night planetarium software. NightSkyInfo.com Diagram

7. How fast will Comet ISON move across the night sky?

Many people who have never seen a comet commonly assume that a comet streaks across the sky at great speed, in the direction opposite to its tail. In fact, when you observe a comet in the sky, there is no rapid motion at all.

Comet ISON will appear to stay in the same place among the stars, having only the motion across our sky caused by the rotation of the Earth. It does move among the stars, of course, but its motion can be seen only over a few days.

As a consequence of Kepler’s second law, Comet ISON’s orbital speed varies according to its distance from the Sun. A comet moves faster and faster as it approaches the Sun, because the closer an object is to the Sun the stronger the Sun’s gravity acts on it.

For example, in mid-2013, ISON will travel at around 50,000 miles per hour (80,000 km per hour). At perihelion on November 28, in contrast, it will move at a whopping 425,000 miles per hour (680,000 km per hour).

8. What is the size of C/2012 S1 ISON?

Observations with NASA’s Swift space observatory in late January 2013 and with the iconic Hubble Space Telescope in April 2013, suggest that C/2012 S1’s nucleus (the solid, central part of the comet, composed of rock, dust and water ice) is 3 miles (5 km) in diameter.

Most comets are no more than 10 miles (16 km) across, so ISON is just the typical size for a comet. When it comes to a comet’s brightness at perihelion, the diameter is not very important. What matters most is how much dust and gas the comet produces and how close it approaches to the Sun.

Many spectacular comets in the past had nuclei much smaller than Comet ISON’s. Perfect examples are Comet Lovejoy, which was just 400 feet (120 meters) across, or Comet Hyakutake, 2.6 miles (4.2 km) in diameter.

9. How bright will Comet ISON be?

Astronomers always had a tough time when it comes to predicting comet behavior. Many “Comet of the century” candidates eventually turned out to be great disappointments. One example is Comet Kohoutek, discovered in 1973 and hyped by the media as a “Great comet”. However, when Comet Kohoutek approached the Sun it partially disintegrated, and fell far short of expectations.

Comet ISON certainly has the potential to reach significant brightness. Its orbit is remarkably similar to that of the “Great Comet of 1680″, which was visible even in daytime and developed a spectacularly long tail. ISON’s intrinsic brightness (also termed absolute magnitude) appears to fit between those of two spectacular comets of the past: C/1965 S1 Ikeya-Seki and C/2006 P1 McNaught. If Comet ISON does not completely disintegrate at perihelion, it should put on a fine naked eye display.

Comet Ikeya-Seki
Comet Ikeya-Seki (pictured above) may be the best “modern-era” equivalent of Comet ISON. Ikeya-Seki reached perihelion on October 21, 1965. Image credit unknown

From mid-northern latitudes, Comet ISON will first appear above the horizon around the second week of August. At that time, it will only be visible with 6 to 8 inch telescopes, maybe at magnitude +10 or +11. Throughout late September and all of October it should be visible with small telescopes or binoculars, and by November 1 the comet may become marginally visible with the unaided eye.

Until its November 28 perihelion, ISON will brighten steadily, maybe to magnitude +1 or +0. This is a brightness similar to that of Arcturus and Vega, the brightest stars in the northern celestial hemisphere. On November 28, when closest to the Sun, the comet could reach magnitude -10, one hundred times brighter than Venus at its brightest! At this time, careful observers may briefly see ISON in the daytime sky next to the Sun.

In a few days after perihelion and maximum brightness, Comet ISON will reappear in the morning twilight, but will have faded to maybe magnitude -1 or +0. However, now it will display an impressive tail, stretching across much of the sky. Mid-December will be the crescendo of the apparition, as the comet will be visible both in the evening and morning twilight. ISON should remain a naked eye sight until late January 2014.

Keep in mind that when it comes to ISON’s brightness anything can happen. The comet could “fizzle” before reaching us, it may fragment, or undergo a spectacular outburst. One thing remains certain; Comet ISON is now speeding toward the Sun. Let us hope for a good show!

10. Is there any link between Comet ISON and the Great Comet of 1680?

Soon after Comet ISON was discovered, astronomers noted that its preliminary orbit had an exceptional similarity with that of the Great Comet of 1680 (formal designation C/1680 V1). Many suggested that the two comets have a common origin and are perhaps fragments of a much larger object, which disintegrated at some point in the distant past.

However, this remains just speculation. Even though both comets have almost identical inclinations (angles with respect to the ecliptic) and small perihelion distances, their orbital eccentricities (amounts by which the orbits around the Sun deviate from a perfect circle) are fundamentally different.

The Great Comet of 1680
The Great Comet of 1680 (also called Kirch’s Comet and Newton’s Comet) was one of the brightest comets of the 17th century, visible during the daytime at perihelion and noted for a spectacularly long tail. It passed only 576,000 miles (927,000 km) from the Sun on December 18, 1680. Painting by Dutch artist Lieve Pietersz Verschuier (1627–1686).

The Great Comet of 1680 has an elliptical orbit, and comes close to the Sun every 10,000 years. Comet ISON, on the other hand, moves on a hyperbolic orbit and makes a single pass into our inner solar system. After its (hopefully) spectacular display in November and December, it will return to the deep freeze of interstellar space, never to be seen again.

Orbit Comparison

For an explanation of these orbital elements, please refer to this Wikipedia page.

The Great Comet of 1680Comet ISON
Epoch: December 18, 1680Epoch: April 18, 2013
Eccentricity: 0.999986Eccentricity: 1.000004
Semi-major axis: 444 AUSemi-major axis: negative (hyperbolic) *
Perihelion distance: 0.00622 AUPerihelion distance: 0.01247 AU
Inclination: 60.7°Inclination: 61.8°
Longitude of the ascending node: 277°Longitude of the ascending node: 295°
Argument of perihelion: 351°Argument of perihelion: 345°
Mean anomaly at the epoch: 359.99°Mean anomaly at the epoch: 359.99°
Date of perihelion passage: December 18, 1680Date of perihelion passage: November 28, 2013
Orbital period: around 10,000 yearsOrbital period: ejection trajectory (one-time visitor)

* Note that the comet’s incoming orbit is nearly parabolic. After planetary perturbations within the solar system, Comet ISON will leave with a hyperbolic orbit.

11. Could C/2012 S1 ISON create a meteor shower?

In mid-January 2014, our planet will pass near the orbit of Comet ISON, and probably through a stream of micron-sized debris left behind by the comet. Some suggested that this encounter brings up the possibility of a meteor shower; however, this is very unlikely.

Most meteor showers occur when the Earth intersects the debris trail of a comet; for instance, the Perseids in early August are associated with Comet Swift–Tuttle, which has a period of 133 years and last returned to perihelion in 1992. The catch is that the Earth actually has to cross a comet’s tail.

On January 14th and 15th, 2014, the Earth does not move through Comet ISON’s tail, it only passes briefly close to its orbit. Assuming that cometary debris is present in ISON’s orbit nearly one hundred days behind the nucleus, the extremely tiny grains of dust will hit the Earth’s upper atmosphere at 35 miles per second (56 km per second).

However, because they are so small, the atmosphere will rapidly slow them to a stop. Instead of burning up in a flash of light, triggering so-called “shooting stars”, they will just drift down to the Earth’s surface.

This invisible rain of comet dust will be so slow, that it will take years for the fine particles to settle out of the high atmosphere. Noctilucent (literally meaning “night shining” in Latin) clouds above the Earth’s polar regions may form from the debris, but only if a significant amount of dust will enter the Earth’s atmosphere. This is also unlikely, as no such events are known to have taken place when Earth has crossed the orbits of other comets under similar circumstances.

Noctilucent Clouds Over Central Asia
In mid-January 2014, the Earth will pass near the orbit of Comet ISON. Micron-sized dust particles shed from the comet could produce noctilucent clouds, but a meteor shower is highly improbable. In the image above, taken from the International Space Station, noctilucent clouds gather over central Asia. Earth’s dark horizon appears below, with some layers of the lower atmosphere already illuminated. NASA

Astronomers have looked in vain for meteor outbursts from other great comets in the past, and meteor showers from one-pass comets coming from the Oort Cloud are very rare. Well-known annual showers are produced by dust ejected from short-period comets, decades or centuries before.

12. Will Comet ISON be visible from Australia and the Southern Hemisphere?

For observers in the Southern Hemisphere, C/2012 S1 ISON will be visible low in the morning twilight in the months leading up to its closest approach with the Sun in late November. Unfortunately, the comet will not be observable at all during the highlight in December.

Throughout August, September and October, Comet ISON will be visible low above the northeast horizon just before sunrise. The best chance to spot it from southern latitudes will be from mid to late November, when the comet should be a naked eye object visible low in the east during early dawn. If ISON will develop a bright tail, it will point upwards, away from the rising Sun.

On November 29 (Australian time), at perihelion, ISON should brighten to naked eye visibility in broad daylight. The comet will be very close to the Sun, so extreme caution will be required if you try to spot it in the daytime sky. Please refer to this section for tips on observing ISON at perihelion.

After November 29, as it will move towards the Earth, Comet ISON will set before the Sun in the evening and rise with the Sun in the morning. It will remain visible only from locations north of the equator.

Viewing Guide

Viewing and photographing Comet C/2012 S1 ISON

For bright comets, the optical instruments of choice are just the naked eyes and, to a lesser extent, binoculars. When Comet ISON will put on its best display, throughout November and December, its coma will look like a tiny ball of light set within a milky glow. From the solid part of the comet, the tiny icy “nucleus” hidden within the coma, the tail (or tails!) will arc across several degrees of sky.

There are two types of comet tails. The gas tail is usually a straight, narrow streamer of light, composed of gas that shines on its own with a bluish tint. If present, a broader dust tail may be the most striking visual feature, appearing as a curving fan of radiance with a yellowish hue. The dust tail is composed of particles previously trapped within the nucleus, which shine by reflecting sunlight.

Comet Hale Bopp
Pictured above, the two tails of Comet Hale-Bopp. The bluish gas tail points directly away from the Sun and shies, like the gas in the coma, by fluorescence. The broader yellow dust tail points nearly away from the Sun, and shies by reflecting sunlight. E. Kolmhofer/H. Raab/Johannes-Kepler-Observatory

When Comet ISON will be low in the bright morning or evening twilight, you will see it much better if the humidity is low and the sky is free of haze. Furthermore, if the comet will develop a long tail that extends far up into the night sky, well out of the twilight, you should travel to a dark-sky site. When far from city lights, you can see the farthest and faintest extensions of the tail.

If you wish to purchase a telescope primarily for observing Comet ISON, you should choose one that can deliver a wide filed of view. A short focus refractor with an eyepiece of large apparent field (55° or more) will afford the best views.

However, keep in mind that these instruments are rather expensive. If you are short on your budget, the simplest, cheapest, and overall the most efficient design you could choose is the Newtonian reflector. An aperture of 6 to 8 inches and a focal ration of f/5 is most appropriate.

Once you have your telescope set on ISON’s coma, the first thing you should do is to try the effect of a wide range of magnifications. Look carefully at the central condensation within the coma – it may look like a hard-edged disk or it may appear as a starlike point of light. This is called the “apparent nucleus”, and it is always larger than the true nucleus (only a few miles across in diameter).

High magnifications may show features called “jets”, coming out from the central condensation. These are fountains of gas and dust, shooting up from the nucleus! Also, look for “halos”, expanding outward form the inner coma, in circular shells.

Now, a few tips on photographing Comet C/2012 S1 ISON. With a modern DSLR camera, equipped with standard lens (35 mm format equivalent, focal length of 50 mm), the exposure should be limited to about 10 seconds. Exposures longer than 10 seconds will produce star trails and smudge the image of the comet.

Be sure to mount the camera on a sturdy tripod and use either a remote shutter control or self-timer. Take several photographs, and if possible take another set an hour or two later to track the comet’s motion relative to the background stars.

Later, you can combine the series of snapshots into one image, using image-processing software. The processing will allow the enhancement of the limiting magnitude and will bring out more detail in the comet’s fuzzy image.

Comet ISON in August 2013

Update (August 18): The comet fell far short of expectations; it glows at magnitude +14 and is only visible with large telescopes.

For observers located in the Northern Hemisphere, Comet ISON will first appear in August’s morning sky. Because throughout the month the comet’s elongation (the angle between the Sun and the comet, with the Earth as the reference point) will be less than 30°, it will be a challenging object, keeping very low above the eastern horizon shortly before sunrise. ISON should be visible in 8-inch telescopes, but it will most probably be just a dim, round fuzz ball, with no hint of a tail.

In early August, from mid-northern latitudes, ISON will rise around 4:30 A.M. local daylight time. By 5:30 A.M., the comet will be just 5° above the east-northeast horizon. The bright twin stars of the constellation Gemini, Castor and Pollux, will shine close to the comet, a little to the north. To the upper right you will be able to see a trio of bright planets – Mercury, Mars, and Jupiter. On August 4, a slender crescent Moon will also join the cosmic scene.

By mid-August, Comet ISON will move a little higher in the predawn sky. Mercury will now be directly below the comet, with Mars and Jupiter remaining to the upper right. The comet will now rise a full two hours before the Sun.

As August closes, ISON will have moved from Gemini into Cancer, and will be located even higher into a darker sky. Mars will shine to its upper right, and the beautiful naked eye star cluster M44 (also known as The Beehive) will be just 2° to the comet’s lower right.

Naked eye map – Comet ISON location in early August – 30 minutes before sunrise, looking east-northeast.

Naked eye map – Comet ISON location in mid-August – 30 minutes before sunrise, looking east-northeast.

Naked eye map – Comet ISON location in late August – 30 minutes before sunrise, looking east.

Telescope map – Comet ISON position throughout August – field width 15°, stars to magnitude +8.

Ephemeris for August

Comet ISON in September 2013

Update (September 5): Several amateur astronomers using large telescopes have reported visual observations of the comet. The estimated magnitude is +12, with a coma one arcminute across. ISON has brightened by two magnitudes since its recovery on August 12, so things begin to look promising!

At the start of September, predawn views of C/2012 S1 ISON will improve, a pattern that will hold throughout the following months. During that time, the comet will slowly brighten and move day by day against the backdrop of the constellation Cancer the Crab. Comet ISON should be visible in small telescopes by September, and dedicated sky watchers will surely try to pick it up.

Seen in the morning twilight early in the month, Comet ISON will be a good 20° above the eastern horizon (10° can be approximated by the width of one’s fist held at arms length). You will be able to search for it with 4-inch telescopes, very close to the 5th-magnitude star Gamma Cancri and the beautiful open star cluster M44, the Beehive. Mars will lie to the upper right of ISON and a waning crescent Moon will also join in on the 2nd and 3rd.

By September 15, C/2012 S1 ISON will be one-third of the way up from the eastern horizon about 30 minutes before sunrise. Mars will lie just 3° to the right and brilliant Regulus (Alpha Leonis) will sit just below.

In late September, Comet ISON will move from Cancer into neighboring Leo. It will lie 215 million miles (346 million km) from the Earth and 163 million miles (262 million km) from the Sun. By that time, astronomers should have a good idea just how much of a phenomenon the comet could turn into.

Naked eye map – Comet ISON location in early September – 30 minutes before sunrise, looking east.

Naked eye map – Comet ISON location in mid-September – 30 minutes before sunrise, looking east.

Naked eye map – Comet ISON location in late September – 30 minutes before sunrise, looking east.

Telescope map – Comet ISON position throughout September – field width 20°, stars to magnitude +8.

Ephemeris for September

Comet ISON in October 2013

Update (October 2): The comet currently glows at around magnitude +10.5, so you should be able to pick it up through a 6-inch telescope under a dark sky. By late October, ISON will be around magnitude +7.5 or +8.

In October, the comet will remain a morning object. Seen in the frigid predawn darkness early in the month, Comet ISON will steadily brighten and slowly move day by day against the background stars of the constellation Leo the Lion. Mars will be very close by, just 2° to the lower right of ISON.

Around October 1, when Comet ISON will make its closest approach – 6.7 million miles (10.8 million kilometers) – to Mars, the scientists at NASA will point the cameras onboard the Curiosity and Opportunity rovers on the comet. Mars Odyssey and the Mars Reconnaissance Orbiter will also snap pictures from orbit.

By October 15, Comet ISON will clear the eastern horizon around 3 A.M. local daylight time. As the morning twilight will start to bathe the eastern part of the sky, the comet will have already climbed to an altitude of 40°. ISON, Mars and Regulus (Leo’s brightest star) will be very tightly grouped, sharing the same telescopic field of view. You should really make an effort to get up early for this stunning celestial scene! The comet may well reach magnitude +5 by mid-October, becoming marginally visible to the unaided eye.

Starting October 15, ISON will enter the field of view of one of the two nearly identical STEREO orbiting solar observatories. The Hubble Space Telescope and the orbiting Chandra X-Ray Observatory will also study the comet during this period, so we should look forward to some amazing pictures!

In late October, C/2012 S1 ISON will have moved to the southern part of Leo and will appear more than 15° to the lower left of Regulus. Mars will sit 5° above the comet, and on the 29th and 30th the crescent Moon will join the scene. Even though ISON will head towards the dawn, to its perihelion in late November, it will still appear high in the southeast one hour before sunrise.

Naked eye map – Comet ISON location in early October – one hour before sunrise, looking east.

Naked eye map – Comet ISON location in mid-October – one hour before sunrise, looking east.

Naked eye map – Comet ISON location in late October – one hour before sunrise, looking southeast.

Telescope map – Comet ISON position throughout October – field width 30°, stars to magnitude +7.5.

Ephemeris for October

Comet ISON in November 2013

Update (November 17): After a major outburst around November 13, Comet ISON is now magnitude +5 and visible with the naked eye from dark sky locations. Chances for a spectacular display in December now seem higher than ever…

The first week of November should be very good for viewing Comet ISON, which by that time might have brightened to naked eye visibility. The comet will still be a pre-dawn object, visible fairly low in the east, below  Mars and 10° to the right of Denebola (Beta Leonis). No matter how cold it will get, you should definitely bundle up, step outside, and have a look at the wonderful addition to the late autumn night sky.

Although no one can know for sure how long ISON’s tail will be in early November, we do know that it will point directly away from the Sun, so it will be streaming in the direction of the planet Mars. If we are lucky, C/2012 S1 ISON will display both a broad dust tail and a wispy blue ion tail. From within city limits, where the sky is not very dark, the comet may appear as a tail-less, fuzzy smudge.

By mid-November, Comet ISON’s path will carry it away from Leo toward Virgo. The comet will have moved well away from Mars (30° to the lower left of the planet) and will have begun its plunge in the Sun’s glare, towards perihelion. Although only 20° above the southeastern horizon around 6 A.M. local time, from urban locations ISON will be easy to spot, and from darker sites it will be impressive.

On November 18, C/2012 S1 ISON will pass very close to Virgo’s brightest star, 1st-magnitude Spica. Both objects will then fit within a 1° circle, and as a bonus, on the same morning, Mercury and the periodic comet 2P/Encke (around magnitude +5 or +6) will be visible just below ISON, close to the horizon. Binoculars will prove useful if not mandatory for spotting Mercury and Comet Encke.

After only a few days, starting around November 21, morning twilight will swallow Comet ISON. However, if the comet will develop a bright tail, there is a slim chance that we might see part of the tail poking above the horizon just before sunrise. Odds favoring that are low, and probably ISON will remain out of sight until closest approach to the Sun on November 28. On that day the comet should return to view – in broad daylight, close to the Sun!

If Comet ISON will reach magnitude -10 at perihelion on November 28, it will be easily visible with the naked eye as a bright speck close to the Sun. If the comet will be dimmer (magnitude -8 to -6), chances are that it will be quite difficult to observe. In either case, because ISON will be only 1.5° away from the Sun’s disk, extreme care should be taken.

Never look directly at the Sun, especially through binoculars and telescopes. It can cause severe damage to the eye! Before attempting to view the comet in daylight, make sure the Sun will be hidden behind a building or a wall.

The following diagrams show the position of Comet ISON relative to the Sun’s disk, at certain times on November 28. The diagrams are drawn for locations in the United States and Europe, and are divided into two tables – one for the United States and one for Europe, respectively.

The Sun’s disk and the separation from the comet are shown to scale, with north at the top of the image and west to the right. Keep in mind that ISON will be very close to the Sun (1.5° on average, around three times the apparent diameter of the Sun). If you will try to observe the comet in daylight, use just your naked eye, no binoculars, telescopes, eclipse glasses or welding glass. Make sure that the Sun is blocked by something solid!

United States

Eastern Standard Time (EST)
Central Standard Time (CST)
Mountain Standard Time (MST)
Pacific Standard Time (PST)
9 A.M.9 A.M.9 A.M.9 A.M.
10 A.M.10 A.M.10 A.M.10 A.M.
11 A.M.11 A.M.11 A.M.11 A.M.
12 P.M.12 P.M.12 P.M.12 P.M.
1 P.M.1 P.M.1 P.M.1 P.M.
2 P.M.2 P.M.2 P.M.2 P.M.
3 P.M.3 P.M.3 P.M.3 P.M.
4 P.M.4 P.M.4 P.M.4 P.M.


Western Europe (UTC)
Central European Time (CET)
Eastern European Time (EET)

After perihelion, C/2012 S1 ISON will rapidly begin to fade, and will remain lost in the Sun’s glare until about December 5. However, the best will still be to come! December will likely be the highlight of Comet ISON’s apparition.

Naked eye map – Comet ISON location in early November – 30 minutes before sunrise, looking southeast.

Naked eye map – Comet ISON location in mid-November – 30 minutes before sunrise, looking southeast.

Naked eye map – Comet ISON location in late November – 30 minutes before sunrise, looking southeast.

Telescope map – Comet ISON position throughout November – field width 90°, stars to magnitude +5.5.

Ephemeris for November

Comet ISON in December 2013

Update (December 1): It now seems unlikely that there will be much to see when the comet (or what is left of it) returns to dawn visibility after December 5. Keep in mid, however, that during most of November, Comet ISON brightened and dimmed in unexpected ways. There may be hope yet.

In early December, the show put on by Comet ISON should really start. Around December 5, the comet will reappear in the eastern morning sky as an unmistakable object visible to anyone who will bother to look. Although after perihelion ISON’s head will have faded to about magnitude +0, the impressive tail should be visible even from light-polluted cities. The tail will grow longer and longer with each passing day, while the comet’s head will become less distinct.

The highlight of ISON’s apparition will certainly occur in mid-December. The comet will then be visible at both dawn and dusk, a third of the way up from the eastern horizon just before sunrise and low in the west soon after sunset. Because Comet ISON makes its first trek in from the distant Oort Cloud, its nucleus should be especially ice-rich, leading to the formation of a long tail. We can only guess just how long it will be, but will likely reach from southern Hercules to the handle of the Big Dipper (almost a quarter of the celestial sphere).

By Christmas week, around the time when it will make its closest approach to Earth, Comet ISON will be circumpolar from mid-northern latitudes. It will stay visible all night, 20° above the northwestern horizon in the early evening and 60° high in the northeast just before dawn. ISON should remain a naked eye sight from dark-sky locations, but will probably require binoculars if observed from the city.

It is helpful to address some common misconceptions regarding comets. Unlike those on TV, real comets do not flash across the sky at some special time of night, nor is the tail a trail that they leave behind. Comets move at about the same speed as planets, so ISON’s visual location among the background stars will not change much from one night to the next.

Second, remember that comets are not especially colorful, at least by comparison with the artificial lights around us. Do not worry if you have never seen a comet before, you will easily recognize Comet ISON! It will be unlike anything else in the sky, appearing as a white or bluish streak of light coming from a diffuse round head about the size of the Full Moon.

Naked eye map – Comet ISON location in early December – 30 minutes before sunrise, looking east.

Naked eye map – Comet ISON location in mid-December – 30 minutes after sunset, looking west.

Naked eye map – Comet ISON location in late December – one hour after sunset, looking north.

Telescope map – Comet ISON position throughout December – field width 160°, stars to magnitude +4.5.

Ephemeris for December

Comet ISON in January 2014

Throughout January 2014, C/2012 S1 ISON will be circumpolar from mid-northern latitudes, meaning that it will never set and will remain visible all night. The comet will start the month at the border between the constellations Draco and Ursa Minor, will sweep by the Pole Star on January 6 and 7 moving in Cepheus, and finally will cross into the large but faint constellation Camelopardalis around mid-month.

Around the same time (January 14 and 15), the Earth will pass near the orbit of Comet ISON, at a distance of only 2.8 million miles (4.5 million km). Because this area of space will probably be littered with a stream of fine-grained debris left behind by the comet, some astronomers have suggested that an odd meteor shower or night-shining noctilucent clouds could occur. However, this seems very unlikely – please see FAQ #11.

Because in January ISON will loop through the solar system in the direction opposite Earth’s motion, our separation will increase rapidly. Early in the month, the comet should still be marginally visible with the naked eye, but by mid-January it will probably dim to magnitude +7 or +8. It will remain a nice target for binoculars and small telescopes, as long as you will be away from city lights and the Moon will lie below the horizon.

Medium to high power will deliver the best views of ISON’s structure. The tail should fade quickly on the western side while the eastern edge will remain well defined due to the solar wind pushing back the comet’s dust envelope. Make sure you are dark-adapted, and use averted vision, looking a bit to the side.

Naked eye map – Comet ISON location in early January – one hour after sunset, looking north.

Naked eye map – Comet ISON location in mid-January – one hour after sunset, looking north.

Naked eye map – Comet ISON location in late January – one hour after sunset, looking north.

Telescope map – Comet ISON position throughout January – field width 90°, stars to magnitude +5.

Ephemeris for January