Summer nights mean late sunsets and lingering twilight, hardly a recipe for great night-sky observing. But this January exceeds expectations with fine views of four bright planets.
Venus punches through the deepening twilight first. The brilliant planet brightens from magnitude –4.4 to magnitude –4.7 during January, far surpassing any other planet or star. Venus reaches greatest elongation on the 10th, when it lies 47° east of the Sun and stands 20° above the western horizon an hour after sunset.
The inner planet’s appearance through a telescope changes dramatically this month. On January 1, Venus shows a disk 22″ across that’s slightly more than half-lit. By the 31st, the world’s disk spans 32″ and appears 39 percent illuminated.
The solar system’s largest planet rides high in the north as twilight fades to darkness. Jupiter lies in Taurus the Bull all month, below the conspicuous Hyades star cluster. Shining at magnitude –2.7, you won’t mistake it for any other object.
Although Jupiter lies much farther from Earth than Venus does, the giant planet’s sheer size means that it currently presents a larger disk. Jupiter’s equator spans 45.6″ at midmonth while its polar diameter measures 42.6″. This 3″ discrepancy appears obvious once you know to look for it. Also look for details in the gas giant’s banded atmosphere and enjoy tracking the never-ending motions of the world’s four bright Galilean moons.
Our next planet resides in the west not far from Venus. Saturn shines at magnitude 1.1 against the dim backdrop of Aquarius. Its slow eastward motion relative to the Water-bearer’s stars is no match for Venus, which quickly catches up with and then passes the more distant planet. The two slide within 3° of each other January 20.
Another favorite with telescope owners, Saturn displays even more polar flattening than Jupiter. The ringed world’s equator spans 16.3″ at midmonth while the polar diameter measures 14.7″. Of course, the rings garner most of the attention. They currently span 37″ and tilt 4° to our line of sight.
As darkness descends, Mars becomes prominent in the northeast. The Red Planet begins the year in Cancer the Crab but moves into Gemini the Twins during January’s second week. This is where it reaches opposition and peak visibility on the 16th, when it shines at magnitude –1.4 and remains visible all night. That evening, it also forms a straight line with the Twins’ brightest stars, Castor and Pollux.
Mars lies 96 million kilometers from Earth at its closest a few days earlier and doesn’t appear as large through a telescope as it does at some oppositions. Its apparent diameter peaks at 14.6″, and its disk should show some subtle dark markings during moments of good seeing.
Mercury doesn’t offer much to observers this month. Those with sharp eyes might spot the magnitude –0.4 planet low in the east-southeast before dawn, but its tiny gibbous disk offers little to those with telescopes.
The starry sky
Astronomers have created many catalogs of non-stellar objects over the past few centuries. Experienced observers likely recognize the New General Catalogue (NGC) and the Index Catalogue (IC), but the most famous one comes to us from French comet-hunter Charles Messier (1730–1817). Messier developed his list to warn observers away from objects that looked like comets but were not.
Messier originally cataloged 103 objects. Later researchers extended it to 109 objects based on observations Messier and his colleague, Pierre Méchain (1744–1804), had made.
Yet Messier did not include every cometlike object, and he naturally excluded those that resided too far south to see from his observatory in Paris. What might Messier have added if he wanted a more complete list that extended to the far southern sky?
To address this question, English astronomy popularizer Sir Patrick Moore (1923–2012) assembled a complementary catalog of 109 deep-sky objects. To avoid confusing his listings with Messier’s, he used his full surname (Caldwell-Moore). Caldwell objects run from C1 to C109 and are organized in order of declination from the north polar to south polar regions of the celestial sphere.
Let’s look at some of the Caldwell objects visible from mid-southern latitudes on evenings at this time of year. I’ll begin with C65, also known as the Silver Dollar Galaxy (NGC 253) in Sculptor the Sculptor. It shows up as an edge-on streak through binoculars. Other Caldwell galaxies in Sculptor are C70 (NGC 300) and C72 (NGC 55).
Cetus the Whale holds two worthwhile Caldwell objects: the 8th-magnitude planetary nebula C56 (NGC 246), also known as the Skull Nebula, and the 9th-magnitude spiral galaxy C62 (NGC 247).
Without a doubt, my two favorite Caldwell objects on summer evenings are C106, the lovely globular cluster 47 Tucanae (NGC 104), which is wonderfully concentrated in its central part, and the spectacular C103, best known as the Tarantula Nebula (NGC 2070) in the Large Magellanic Cloud. The latter’s complex spiderlike pattern looks delightful through a 20-centimeter scope.
Many other Caldwell objects lurk in the southern sky. I’ll return to them in future columns when they climb higher in the evening sky.
Star Dome
This map portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
11 p.m. January 1
10 p.m. January 15
9 p.m. January 31
Planets are shown at midmonth
