The year begins with the three other terrestrial (rocky) planets of our solar system—Mercury, Venus and Mars—low in the west-southwest to southwest during evening twilight. Their span shrinks from 27 degrees on Jan. 1, to less than 20 degrees Jan. 11-20.
Bright Venus, of magnitude -3.9, draws attention to the gathering. Find Mercury just to its lower right, within 3 degrees Jan. 1-17, 2 degrees Jan. 4-15, and 1 degree Jan. 8-12. On Jan. 10 at dusk, Mercury appears within two-thirds of a degree to the lower right of Venus! As seen from Earth, Mercury will not overtake Venus, and will instead fall just short. This is a quasi-conjunction, an approach within 5 degrees without an actual conjunction, when two planets share the same “x-coordinate.” Mercury shines at magnitude -0.8 Jan. 1-11, fades through magnitude 0 on Jan. 19, magnitude +1 on Jan. 22, and magnitude +2 on Jan. 24, en route to inferior conjunction Jan. 30. Mars glows at magnitude +1.1 to +1.2, 24 degrees to 10 degrees to the upper left of Venus.
Bright Jupiter,of magnitude -2.5 to -2.6, rises in the east-northeast within 3.2 hours after sunset on Jan. 1, closing to within four-tenths of an hour after on Jan. 31. One hour before sunrise, Jupitergleams in the west-southwest, 8 to 12 degrees west of Regulus, while Saturn glows at magnitude +0.6 to +0.5 in the southeast to south-southeast, 11 to 9 degrees northwest of Antares.
As for the moon: In the evening sky, our satellite “leapfrogs” past Aldebaran from Jan. 1 to Jan. 2. Full moon occurs on evening of Jan. 4, and at dusk, it appears low, north of east, to the right of the Gemini Twins, Pollux and Castor. At next day’s dawn, on Jan. 5, the moon is low, north of west, to the lower left of the Twins. On the night of Jan. 7-8, from late evening to dawn, the waning gibbous moon keeps company with bright Jupiter. Regulus, heart of Leo, is nearby, 9 degrees east of the planet.
For two weeks after full, the waning moon can be followed in the mornings, about an hour before sunrise. The last quarter moon—half-full and 90 degrees, or one-quarter circle, west of the sun—appears very close to Spica on the morning of Jan. 13.
In mid-January each year, our Spaceship Earth heads toward Spica. In the coming months, as Earth curves around the sun, we will overtake the slower-moving outer planets: Jupiter in early February 2015, and Saturn after the middle of May. As a result, these planets, along with their background stars, will progress toward the western horizon of our morning sky, and will appear above our eastern horizon in the early evening sky as Earth passes each one in turn.
On the morning of Jan. 16, a waning crescent moon appears quite close to Saturn. Note the star Antares, heart of the Scorpion, 10 degrees below. On the next morning, Saturday, Jan. 17, the moon will appear in the southeast, widely left of Antares and to the lower left of Saturn. On Sunday, Jan. 18, the moon is still in easy view, 24 degrees to the lower left of Antares. On Monday, Jan. 19, the last old crescent moon is just 3 degrees up in mid-twilight, 23 hours before the invisible new moon of Jan. 20 at 5:14 a.m.
After new, the moon returns to the evening sky. At dusk on Wednesday, Jan. 21, you’ll have your first chance to observe the thin young moon, 37-38 hours after new, in close company with Venus and Mercury. Mars appears to their upper left as twilight fades. On the next night, Thursday, Jan. 22, the crescent will appear near Mars.
First quarter (evening half-moon) is reached on Monday, Jan. 26. As we look into the evening sky in the direction of the first quarter moon (below the bright stars of Aries, the Ram, and two days’ moon travel west of the Pleiades star cluster), we are facing back toward our “wake,” in the opposite direction of the motion of Spaceship Earth around the sun. Between the evenings of Jan. 28 and 29, for the second time this month, the waxing gibbous moon again “leapfrogs” over Aldebaran, eye of the Bull and follower of the Pleiades. It takes about 27.3 days for the moon to return to the same star field, but, because of the Earth’s revolution around the sun, it takes about 29.5 days until the moon repeats the same phase.
The next full moon will occur on Feb. 3, with Jupiter nearby all night.
The two brightest planets, Venus and Jupiter, will be visible simultaneously in the early evening from late January through July. On Jan. 18, Jupiter rises in the east-northeast as Venus sets in the west-northwest. On Jan. 20, near the end of twilight, they both appear about 1 degree up. Look just more than a minute earlier each day to catch these brightest planets equally above opposite horizons: 3 degrees up on Jan. 23; 6 degrees on Jan. 28; and 8 degrees on Jan. 31.
Looking ahead, Venus and Jupiter will gradually come together, attracting the attention of even casual observers, culminating with a spectacular close pairing of Venus and Jupiter on the evening of June 30, when they’ll appear just one-third of a degree apart in the western sky. (Do not miss the telescopic view!) The five-month run-up from late January to June 30 provides a fine chance to follow these planets and watch for changes. Not until 2035-2036 will there be such a long an interval to follow Venus and Jupiter in the sky together on their way to a brilliant pairing.
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller, who provided the twilight charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University. He remains active in research and public outreach in astronomy.