The Planets


Mercury, the innermost and smallest planet of the solar system, hugs the southwestern horizon right after sunset. Spotting it could prove quite challenging, but if you plan ahead and observe from a location with a good horizon without obstructions (such as trees or buildings), you should be able to see it. The best chance comes around late December; greatest elongation occurs on the 28th and this is when Mercury is highest in the sky.

Of the famous five naked-eye planets, Mercury can sometimes be the second brightest, outshining all stars, and can often be the planet closest to Earth – and yet it has been seen by remarkably few people! This is because Mercury, the innermost planet of the solar system, can never depart more than 28° from the Sun as seen from our point of view.

Mercury can never be seen more than roughly two hours before or after the Sun, so because of this it is not well known. The beautiful, and sometimes orange planet, can be amazingly prominent near greatest elongation at its few best apparitions of the year (there are many separate apparitions of Mercury in a year because the swift planet races back and forth between the morning and evening sides of the Sun).

To know when one of these favorable periods will occur (this month’s apparition is far from favorable) please consult this page at the beginning of each month. However, observers at mid-northern latitudes may follow the general rule that Mercury is best seen as an evening object sometime in late winter or early spring, and as a morning object in late summer or early autumn.

Finder map (mid-December) – 30 minutes after sunset, looking southwest.
Finder map (late December) – 30 minutes after sunset, looking southwest.


Venus is the brilliant “Morning Star”, rising about two hours before the Sun and dominating the southeastern sky until sunrise. At magnitude -4.2, the planet is five full magnitudes (a factor of 100) brighter than its companion Spica, which stands just 4° away in early December.

Venus moves quickly across the starry background, crossing into Libra on the 11th. It passes double star Alpha Librae on the 17th and 18th, and pulls very close to the telescopic double star Beta Scorpii at month’s end.

For such a dazzling naked-eye sight, Venus is one of the most profoundly disappointing sights in the telescope. The planet’s surface is perpetually obscured by an extremely thick atmosphere, so any observations will be limited to details discernible in its rather bland covering – slightly brighter or darker areas, or irregularities in the terminator (the line that separates the illuminated day side and the dark night side of the planet).

It has been found that the top of the atmosphere lies around 250 miles (402 kilometers) above the surface, and that the upper clouds have a rotation period of only four days. The upper clouds lie at an altitude of 45 miles (72 kilometers), and there are several definite cloud-layers, though below 20 miles (32 kilometers) the atmosphere is relatively clear and calm.

The atmosphere’s main constituent is carbon dioxide, accounting for over 95-percent of the whole; most of the rest is nitrogen. The clouds are rich in sulfuric acid; at some levels, there must be sulfuric acid “rain” which evaporates before reaching ground level.

Finder map (early December) – one hour before sunrise, looking southeast.
Finder map (mid-December) – one hour before sunrise, looking southeast.
Finder map (late December) – one hour before sunrise, looking southeast.


A Small Portion of Acidalia Planitia
A small portion of Acidalia Planitia, a largely flat plain that is part of Mars’ vast northern lowlands. ESA / The Mars Express Team

Mars continues to flee ahead of Earth in orbit around the Sun. Nevertheless, our world is slowly gaining on the Red Planet, and, as we do so, Mars climbs higher each day into the southeastern sky before dawn.

Mars starts December near 3rd-magnitude Gamma Virginis and brushes just past 4th-magnitude Theta Virginis on December 12 and 13. On the mornings of December 23 and 24, it passes about 4° north (upper left) of bright Spica.

Mars lies about 171 million miles (275 million kilometers) from Earth around mid-month. Coupled with the planet’s small physical size, this large distance renders the orange-gold world tiny in the eyepiece, just 5” across. Only the largest backyard telescopes will show surface features with any clarity.

Under good conditions, an amateur telescope will show Mars’ polar ice caps and the main dark areas. These were once thought to be seas but now are known to be regions where winds in the tenuous atmosphere have blown away the red, dusty material that covers most of the planet, exposing the darker layers below.

The two most prominent dark surface markings are the Syrtis Major in the equatorial region of the planet and Acidalia Planitia in the north; the Syrtis Major is V-shaped, and much the easiest to see. Do not expect to be able to make out a lot on Mars the first time you look through your telescope. Observing takes lots of practice, and you will find that many hours of experience will make an enormous difference to what you can make out.

Finder map (early December) – one hour before sunrise, looking southeast.
Finder map (mid-December) – one hour before sunrise, looking southeast.
Finder map (late December) – one hour before sunrise, looking south.


The Galilean Moons
The four Galilean moons. From left to right, in order of increasing distance from Jupiter: Io, Europa, Ganymede, and Callisto. NASA / JPL / DLR

Jupiter rises in the east around 12:30 A.M. local time in early December and more than an hour earlier by month’s end. Before dawn it stands 50° high in the southeast, among the stars of Leo the Lion. During the month, the giant planet brightens from magnitude -2 to -2.2 and in telescopes grows from about 36” to 39” wide.

Telescopically, Jupiter’s belts, zones, spots, rifts, and other markings invite night after night of study. The planet’s four Galilean moons – Io, Europa, Ganymede and Callisto – are also easy targets for beginners with small telescopes. These moons orbit Jupiter so quickly, that their motion can be seen almost minute by minute if they are next to the planet or to each other.

This Jupiter’s Moons diagram shows where they lie with respect to the planet at any time and date this month (at midnight Universal Time). To convert your time and date to Universal Time for using the chart, subtract the following hours: EST, 5; CST, 6; MST, 7; PST, 8; Alaska, 9; or Hawaii, 10. The result is on the date before the Universal Time date given. Watch out – to match the view through a telescope, turn the diagram with north at the bottom. If you use binoculars, north should be at the top.

Have you ever looked for Ganymede and Callisto with your unaided eye? The two moons are within reach at their eastern and western elongations from Jupiter, if you have superb vision. It helps to block Jupiter with the edge of a building or tree limb. The diagram suggests when to try.

Finder map (early December) – 5 A.M. local time, looking southeast.
Finder map (mid-December) – 4 A.M. local time, looking southeast.
Finder map (late December) – 3 A.M. local time, looking southeast.


Saturn and Six of its Moons
Pictured above, Saturn and six of its moons. Titan appears at the lower left and continuing to the right are Mimas, Tethys, Enceladus, Dione, and Rhea. Rafael Defavari

Saturn, having passed through conjunction with the Sun on November 29, becomes plainly visible around mid-December, when it rises in the southeast more than an hour before sunup.

The ringed planet shines at magnitude +0.5 and should show up with the naked eye for observers with a haze-free and unobstructed horizon. If you cannot spy it right away, binoculars will bring it into view. Do not confuse Saturn with Beta Scorpii, about 7° to the northwest, or with twinkling Antares 6° to the southwest.

Saturn orbits the Sun at an average distance of 9.6 astronomical units (AU); its distance from the Earth varies from about 8.5 AU to 10.5 AU. It takes Saturn nearly 30 Earth years to complete one revolution around the Sun, so a year on Saturn is 30 times longer than a year on Earth.

Like all planets, Saturn rotates on its axis – an imaginary line running through a planet from its north pole to its south pole. In some planets, the axis is almost vertical, or upright, in relation to the planet’s orbit. But in Saturn, the axis is tilted at 26.73°, causing different parts of the planet to lean closer to the Sun.

The same thing occurs on Earth and brings about the regular changes in the weather we call the seasons. Because Saturn takes a lot longer to orbit the Sun, its seasons last for 7.5 years.

Finder map (mid-December) – 30 minutes before sunrise, looking southeast.
Finder map (late December) – 30 minutes before sunrise, looking southeast.


Uranus appears slightly more than halfway to the zenith in the southeast as darkness falls and remains on view all evening. It does not set until around 1 A.M. local time, even as December comes to a close. The seventh planet lies among the background stars of Pisces the Fish, 2° south of 4th-magnitude Epsilon Piscium.

Use the Great Square of Pegasus as a guide to Uranus’ general location. Start by drawing an imaginary line from Beta to Gamma Pegasi, the top right corner and bottom left corner of the asterism, respectively, on December evenings. Then continue the line and head slightly left to pick up Epsilon Piscium.

Uranus is not one of the five classic naked-eye planets, but observers under dark skies should still be able to glimpse the first “discovered” planet without optical aid. It remains at magnitude +5.8 throughout the month.

If you use a telescope at high magnification, on a night with good seeing conditions, Uranus appears as a tiny featureless disk with a pale greenish hue. The planet’s 27 known natural satellites are beyond easy reach of most amateur telescopes, for visual observations.

However, if you are equipped with one of the very large-aperture telescopes that have become common nowadays you will find it possible to glimpse the largest and brightest of the moons. Titania (magnitude +13.9) and Oberon (magnitude +14.1) will be the easiest because they attain the greatest separation from the glare of the planet.

Finder map – field width 15°, stars to magnitude +8.


Neptune from Voyager2
Bluish Neptune, the solar system’s outermost gas giant, is a dynamic planet with several large, dark spots reminiscent of Jupiter’s hurricane-like storms. NASA / JPL / Voyager 2

Uranus’ outer neighbor, Neptune, stands high in the south as soon as darkness falls this month. Look for it in Aquarius the Water-Bearer, roughly one-third of the way from 5th-magnitude Sigma Aquarii to 4th-magnitude Lambda. Try to catch a view before 8 P.M. local time, because Neptune becomes harder to see as its altitude declines. The distant planet sets by 11 P.M. local time.

Neptune glows at magnitude +7.9, bringing it within range of binoculars. It is much easier to see, however, if you mount the binoculars on a tripod or use a small telescope. The planet’s blue-gray disk appears 2.4” across, just big enough to be resolved under good conditions.

The atmospheric composition of Neptune is similar to that of Uranus. Compared to Uranus, Neptune appears bluer, presumably due to a higher concentration of methane (around 3-percent) in the atmosphere.

Neptune’s Great Dark Spot was first observed by Voyager 2 in the Southern Hemisphere in 1989. This storm disappeared by the time the Hubble Space Telescope observed Neptune in 1994, but a new one had formed in the planet’s Northern Hemisphere by 1995.

Neptune’s interior consists of hydrogen, water, and a small (Earth-sized) core of rock and iron. The giant planet releases internal energy, driving supersonic winds to speeds of over 1,200 miles (2,000 kilometers) per hour. The exact source of the released energy is unclear, but it does not appear to be a remnant from the formation of the planet.

Finder map – field width 15°, stars to magnitude +8.5.


The dwarf planet is not currently observable. It will return to view in February 2016, low in the morning sky.

The Deep Sky

Star Cluster M34
Easy to appreciate in binoculars or small telescopes, M34 lies some 1,400 light years away in the constellation Perseus. Bob Franke

Perseus, a son of Zeus, is a member of winter’s “royal family”, which also includes Andromeda, Cassiopeia and Cepheus. According to the popular legend, Perseus rescued the beautiful maiden Andromeda, chained to a rock as a sacrifice to Cetus the sea monster. For his valor, Perseus was placed among the stars for eternity.

Skywatchers in the Northern Hemisphere see Perseus standing high in the winter sky, in a sparkling region of the Milky Way. The constellation hosts a magnificent array of deep sky wonders, with many fine open star clusters and nebulae.

However, despite this rich assortment of deep sky objects, including the famous Double Cluster, only two objects in Perseus – the open cluster M34 and the planetary nebula M76 – are logged in the famous catalog of 18th-century French comet hunter Charles Messier.

M34, this month’s deep sky highlight, is easy to find with binoculars and can be glimpsed with the naked eye under ideal sky conditions. The open cluster makes an isosceles triangle with Kappa Persei and Beta Persei, or Algol, the remarkable eclipsing binary star whose brightness fades for a few hours every 2.87 days.

Glowing at magnitude +5.5, M34 holds well over 60 stars within its gravitational grip. About a dozen of these suns shine brighter than 9th magnitude and can be resolved with 7×35 binoculars; several are white giants. What looks like the brightest star of the cluster shines at magnitude +7.3, but this star is in the foreground, not a true cluster member. One of the brightest true members is a double star known as Struve 44, whose 8.4- and 9.1-magnitude components are separated by 1.4”.

M34 spreads out over an area roughly 35’ across, a bit bigger than the Full Moon. It has a diameter of about 10 light years and its stars rotate at rates that are midway between those in the younger Pleiades Cluster (100 million years old) and the older Hyades Cluster (600 million years old).

This is thought to be the result of rotational braking whose effect on stellar rotation rates becomes more pronounced with age. Such braking is believed to be due to angular momentum loss via magnetic coupling to the chromosphere (i.e. the star’s atmosphere outside the bright photosphere).

Finder map – field width 15°, stars to magnitude +8.


Vesta’s South Pole
Vesta shows light and dark features, hills, craters and cliffs, much like our Moon. The Dawn spacecraft took this image of the asteroid’s south pole from a distance of about 1,700 miles (2,700 kilometers). NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Asteroid 4 Vesta is the second-most-massive object in the asteroid belt after the dwarf planet Ceres, surviving from the earliest phases of solar system history. It formed at a time when the asteroid belt was much more massive than it is today and was witness to its dramatic evolution, where planetary embryos were formed and lost, and where the collisional environment shifted from accretional to destructive.

In spite of being a bit too small, Vesta could easily be considered the sixth Earth-like planet, in addition to Mercury, Venus, Earth, the Moon, and Mars. Shortly after its formation more than 4 billion years ago, molten lava penetrated the surface, cooled again, and has not changed since. In the various high-resolution images from NASA’s Dawn spacecraft we see one of the oldest surfaces in the solar system.

In contrast, the two asteroids visited by the Galileo probe, Ida and Gaspra, were broken off larger bodies by collisions only several hundred million yeas ago; they are more indicative of the geological present in the asteroid belt.

Even though Vesta reached opposition and peak visibility in late September, it still shines below 8th-magnitude this month and remains well placed for Northern Hemisphere observers. You can find it high in the south during midevening, within easy reach of a 4-inch telescope.

Your signpost to Vesta is magnitude +3.5 Iota Ceti, which itself lies 11° northwest (to the upper right) of 2nd-magnitude Beta Ceti. The asteroid spends the month within a couple of degrees of Iota, sliding from west to northeast of the star. Avoid trying for Vesta on December 18, when the First Quarter Moon will be just 7° away.

Finder map – field width 15°, stars to magnitude +8.5.


C/2013 US10 Catalina was discovered on October 31, 2013, using the 27-inch (0.68 meter) telescope at the Catalina Sky Survey facility near Tucson, Arizona. Initially categorized as a very large near-Earth asteroid (hence its unusual “US10” designation), new observations soon indicated that US10 Catalina was, in fact, a long-period comet from the distant Oort Cloud.

For much of 2015, the comet remained the province of Southern Hemisphere observers. In late July and early August it was slightly brighter than 8th-magnitude, and became a south circumpolar object. Throughout September and October it reached magnitude +6.5, just shy of naked eye visibility.

Now, after making a hairpin turn around the Sun at perihelion on November 15, C/2013 US10 Catalina sails into Northern Hemisphere predawn skies. Although there has been a lot of buzz about the comet, since astronomers were hopeful that it will reach at least 4th magnitude, expectations have been managed downwards recently. C/2013 US10 currently glows at only 6th-magnitude, just at the edge of visibility in a dark sky without optical aid.

Throughout the month, Catalina pursues a northerly track through Virgo and Bootes. On December 1, it appears 15° high in the southeastern sky 60 minutes before sunrise. Imagers should be prepared one week later, the morning of December 7, to capture Comet Catalina sharing an 8° field with Venus and a thin crescent Moon.

After December 8, two weeks of Moon-free skies will let observers get detailed views of the comet’s dust and gas tails. The Moon returns to the morning sky in late December, as Catalina crosses from Virgo into Bootes on a beeline for Arcturus. By then, the comet likely will have faded to 7th magnitude.

Finder map – field width 60°, stars to magnitude +6.

The PanSTARRS Facility
The PanSTARRS PS1 and PS2 telescopes, located at the summit of Maui’s Haleakala volcano. Comet C/2014 S2 was discovered from this facility on September 22, 2014. PS1 Science Consortium / University of Hawaii Institute for Astronomy

Comet PanSTARRS (C/2014 S2) should show up nicely through a 4-inch telescope under a country sky. Glowing around 9th magnitude and with a predicted diameter of five arcminutes, this visitor from the solar system’s icy depths likely will appear similar in brightness and size to some of the brighter elliptical galaxies in the Messier catalog.

The comet belongs to both the evening and morning skies this month because it lies among the background stars of Draco, a circumpolar constellation (that is, never setting) for many observers in the Northern Hemisphere. To see it best, try to sidestep moonlight. In early December, dark skies come in early to mid-evening before the Moon rises. During the month’s second half, the waxing Moon demands a switch to predawn hours.

As December begins, PanSTARRS appears 25° south of Polaris and just 3° southwest of Zeta Draconis, also known as Nodus III (the third of the twists or “nodes” in the tail of the Dragon). Throughout the month, the comet continues to move south from this point; by the 31st it lies 4° east of 3rd-magnitude Eta Draconis.

Finder map – field width 10°, stars to magnitude +9.


The Geminid meteors – one of the year’s most prominent showers – radiate from near Castor, the second brightest star in the constellation Gemini. Many seasoned meteor watchers believe the Geminid shower is better than the August Perseids, and for a good reason: the Geminid radiant is nearly as far from the Sun as one can get in mid-December. As a result, the radiant rises high enough above the horizon only a few hours after sundown and remains high for the rest of the night.

The Geminids are active from December 4 to 17 and peak very quickly on the night of December 13 – 14 (just three days after New Moon). Most activity occurs after midnight on the 14th, when as many as one hundred slow, graceful Geminids might be seen per hour under ideal conditions.

The orbit of the Geminid stream is very small (the revolution period for an object on it is just 1.6 years), but the parent comet has not been found. In 1983, an asteroid-like object named 3200 Phaeton was discovered in the same orbit as the Geminids, and observations have pointed to the fact that Phaeton is actually the shower’s parent body.

Map – Geminids radiant position.

The Ursid shower is active from December 17 to 26, and peaks on the morning of December 23. The radiant lies near the bright star Kochab (Beta Ursae Minoris), which appears below the Pole Star in the evening and above it before dawn. The radiant is circumpolar from most of the Northern Hemisphere, so viewing can last all night.

The Ursids usually produce fairly modest rates, with a typical peak ZHR (Zenithal Hourly Rate) of about ten. In some years, however, the ZHR has exceeded 50. Notably, rates were high in 1945, and again in 1982 and 1986. Unlike those of the Perseids and Leonids, Ursid outbursts do not seem to correlate with the perihelion of the parent comet, 8P Tuttle.

Most Ursid meteors are faint and of slow to medium speed. They can appear anywhere in the sky, although their trails will point back toward the shower’s radiant, in the bowl of the Little Dipper. Because the radiant is so close to the north celestial pole, Ursid meteors are practically nonexistent south of the equator.

Map – Ursids radiant position.

Some meteors do not belong to any known shower. These are the sporadic meteors, caused by random bits of comet debris spread throughout the inner solar system. They appear randomly across the sky all year long.

In this month’s night sky, careful observers can expect around eleven sporadics per hour during the morning hours and three during the dark evening.

Observing Aids

Northern Hemisphere’s Sky – This map portrays the sky as seen near 40° north latitude at 8 P.M. local time in early December and 7 P.M. in late December.

Southern Hemisphere’s Sky – This map is plotted for 35° south latitude. It shows the sky at 10 P.M. local time in early December and 9 P.M. in late December.

Visibility of the Planets – The table provides general information about the visibility of the planets during the current month.

Phases of the Moon – This Moon Phase Calendar shows the Moon’s phase for every day in December.

Jupiter’s Moons – The diagram shows the positions of Galilean satellites on each day in December at midnight.

Sky Events

December 3 – 2:40 A.M. EST: Last Quarter Moon.

December 4 – 1 A.M. EST: The Moon is 1.8° south of Jupiter.

December 5 – 9:56 A.M. EST: The Moon is at apogee, the point in its orbit when it is farthest from Earth. 10 P.M. EST: The Moon is 0.1° south of Mars.

December 7 – 1 P.M. EST: The Moon is 0.7° north of Venus.

December 9 – 9 A.M. EST: Asteroid 16 Psyche is at opposition.

December 11 – 5:29 A.M. EST: New Moon.

December 14 – 5 A.M. EST: The Geminid meteor shower is at peak activity.

December 17 – 3 A.M. EST: The Moon is 3° north of Neptune.

December 18 – 10:14 A.M. EST: First Quarter Moon.

December 19 – 8 P.M. EST: The Moon is 1.2° south of Uranus.

December 21 – 4:00 A.M. EST: The Moon is at perigee, the point in its orbit when it is nearest to Earth. 7 A.M. EST: Mars is 4° north of Spica. 11:48 P.M. EST: The Winter Solstice occurs.

December 23 – 5 A.M. EST: The Ursid meteor shower is at peak activity. 3 P.M. EST: The Moon is 0.7° north of Aldebaran.

December 24 – 1 A.M. EST: Asteroid 27 Euterpe is at opposition.

December 25 – 6:11 A.M. EST: Full Moon.

December 26 – 6 A.M. EST: Uranus is stationary.

December 28 – 10 P.M. EST: Mercury is at greatest eastern elongation, 20° east of the Sun.

December 31 – 1 P.M. EST: The Moon is 1.5° south of Jupiter.

The information provided on this page is accurate for the world’s mid-northern latitudes. Finder maps for the five naked eye planets are plotted for 40° north latitude, but can also be used from other latitudes close to 40° north. Except the two all-sky maps, all other maps can be used no matter the latitude. Local time (local daylight time during summer) represents the time of the reader.

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