Some daily events in the changing sky for January 18 26.
Comets Holmes and Tuttle are far apart in the moonlit evening sky. Use binoculars to try for both. Holmes continues enlarging and dimming, so the moonlight will affect it particularly severely. Holmes crosses the 2nd-magnitude star Algol for several days centered on January 21st; see chart. (The dates on the chart are for 0:00 Universal Time, which falls on the evening of the previous date in the time zones of the Americas.)
Tuttle is much smaller than Holmes, more concentrated, and fading this week from about magnitude 6.0 to 6.5. It's now far south, crossing from southernmost Cetus into Fornax. See the article and chart in the January Sky & Telescope, page 73, or the brief version online. (Again, dates on the chart are for 0:00 UT.)
Tuttle a contact binary? Astronomers have used the Arecibo planetary radar to get an image of Comet Tuttle's solid nucleus with 300-meter resolution. J. K. Harmon and colleagues find that the nucleus "is a strongly bifurcated object, possibly a contact binary, with two roughly spherical lobes measuring 3 and 4 km in diameter (+/- 25 percent)," according to IAU Circular #8909. Also, "following the changing rotation aspect from night to night and within the 2.5-hour observing sessions gives a preliminary estimate of 7.7 +/- 0.2 hours for the rotation period."
Friday, January 18
Saturday, January 19
Sunday, January 20
Monday, January 21
Tuesday, January 22
Wednesday, January 23
Thursday, January 24
Friday, January 25
Saturday, January 26
Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly foldout map in each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standard is Sky Atlas 2000.0) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the even more detailed Night Sky Observer's Guide by Kepple and Sanner, or the enchanting though increasingly dated Burnham's Celestial Handbook). Read how to use them effectively.
More beginners' tips: "How to Start Right in Astronomy".
This Week's Planet Roundup
Mercury (magnitude 0.7) is having a fine evening apparition low in the western twilight. Look for it above the west-southwest horizon 45 to 60 minutes after sundown. Don't confuse it with dimmer, twinklier Fomalhaut far to the left in the southwest. The Messenger spacecraft flew by Mercury on January 14th, sending back our first close looks at the planet in nearly 33 years! See article.
Venus (magnitude 4.0, moving from Ophiuchus into Sagittarius) is the bright "Morning Star" low in the southeast before and during dawn, as shown at right. Look for much dimmer Antares sparkling orange-red off to its upper right, and Jupiter closing in on it day by day to its lower left.
Mars (about magnitude 1.0, in eastern Taurus) shines high in the east to southeast after dark. The blue-white star close above Mars is Beta (β) Tauri, also named El Nath. Mars is highest toward the south around 10 p.m., high over Orion in fact passing near the zenith for observers at mid-northern latitudes.
Mars this week diminishes from 13.6 to 12.7 arcseconds in apparent diameter; it's falling farther behind us as Earth moves ahead of it in our faster orbit around the Sun. Mars is also turning gibbous again. See the telescopic observing guide and surface-feature map in the November Sky & Telescope, page 66, or the short version online.
Jupiter (magnitude 1.8, in Sagittarius) is low in the glow of dawn, lower left of brighter Venus, as shown above. Look for it about 45 minutes before sunrise. (To find your local sunrise time, make sure you've put your location and time zone into our online almanac.)
Once you've found Jupiter, watch it get higher and easier each morning as it closes in on Venus. On January 19th they're still 13° apart, but the gap between them is narrowing by 1° per day. These two brightest planets are heading toward a close conjunction (0.6° apart) on the morning of February 1st.
Saturn (magnitude +0.6, in Leo) rises in the east around 8 p.m. and is highest in the south in the early-morning hours. Fainter Regulus (magnitude +1.4) is 8° west of Saturn: to its upper right after they rise. Only a little dimmer than Regulus is Gamma (γ) Leonis, magnitude +2.1, 8° to Regulus's north. The three make an eye-catching triangle. Regulus and Gamma Leonis are the brightest stars in the Sickle of Leo (as shown in the "January 23" image above).
Uranus (magnitude 5.9, in Aquarius) is low in the southwest right after dark.
Neptune is hidden in the glow of twilight (near Mercury this week).
All descriptions that relate to your horizon or zenith including the words up, down, right, and left are written for the world's midnorthern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Standard Time (EST) equals Universal Time (UT, UTC, or GMT) minus 5 hours.
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