FRIDAY, DECEMBER 9
■ Orion clears the eastern horizon as early as 7 p.m. now (depending on how far east or west you live in your time zone). Every week, the stars and constellations reach a given position in your sky about a half hour earlier. To be exact, 27.6 minutes earlier.
■ The waning gibbous Moon this evening forms a big, nearly equilateral triangle with Mars high to its upper right and Capella high above the Moon.
■ Algol dips to its minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours tonight centered on 12:08 a.m. EST; 9:08 p.m. PST. Algol takes several additional hours to fade and to rebrighten. Comparison-star chart.
SATURDAY, DECEMBER 10
■ The waning gibbous Moon rises in late twilight. Once the stars come out, spot Pollux left of the Moon. Above Pollux is Castor.
They travel together through the night. By early dawn on Sunday the 11th they're way over in the west, oriented with the stick-figure Twins now upright as shown below. And the Moon has crept east along its orbit to line right up with Pollux and Castor.
SUNDAY, DECEMBER 11
■ By 9 p.m. the waning gibbous Moon is well up in the east-northeast, below Castor and Pollux. If you're in North America, the Moon forms an almost perfectly straight line with Procyon, the Little Dog Star about two fists to its right or lower right, and Sirius, the big Dog Star a slightly greater distance beyond in the same direction.
MONDAY, DECEMBER 12
■ You've got about two hours of dark sky now between the end of twilight and moonrise. Here's a project. The Cassiopeia W hangs very high in the northeast after dark. The bottom star of the W is Epsilon (ε) Cassiopeiae, the faintest. That's your starting point for hunting down the little-known, little-observed but unique-looking star cluster Collinder 463. It's sparse, loose, subtle, but visible in large binoculars and wide-field scopes on dark nights. It's 8° to Epsilon's celestial north (the direction toward Polaris, by nearly twice the length of that last segment of the W) and is surrounded by a nice quadrilateral of 4th- and 5th-magnitude stars about 3° wide. The cluster appears nearly 1° long, curved and narrow, with the curve open to the northeast. Its brightest stars are only 8th and 9th magnitude. Use Chart 1 of the Pocket Sky Atlas. Good luck.
■ Algol should be at its minimum brightness for a couple hours centered on 8:57 p.m. EST. Comparison-star chart.
TUESDAY, DECEMBER 13
■ The Geminid meteor shower should be at its peak in the early-morning hours of Wednesday, but by then the light of the waning gibbous Moon will interfere. The Moon rises around 9 or 10 p.m. (with Regulus a few degrees to its lower right). Before that, you will have a dark sky to watch for an occasional long Geminid skimming across the upper atmosphere; the shower's radiant, near Castor in Gemini, will still be fairly low. Be patient.
WEDNESDAY, DECEMBER 14
■ Right after dark in the Eastern time zone, examine Algol in Perseus high in the east. It should be at its minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 5:46 p.m. EST. Comparison-star chart. Keep checking in on it through the evening; Algol takes several hours to rebrighten.
■ Jupiter's Great Red Spot should transit the planet's central meridian around 8:11 p.m. EST. A light blue or green filter at the eyepiece helps a bit.
THURSDAY, DECEMBER 15
■ The Summer Triangle sinks lower in the west as evening proceeds, and Altair is the first of its stars to go (for mid-northern skywatchers). Start by spotting bright Vega, magnitude zero, the brightest star in the northwest soon after dark.
The brightest one above Vega is Deneb. Altair, the Triangle's third star, is farther to Vega's left. How late into the night, and into the advancing season, can you keep Altair in view?
FRIDAY, DECEMBER 16
■ The little Pleiades cluster shines high in the southeast after dinnertime, upper right of Mars by about a fist at arm's length. As shown below. It's no bigger than your fingertip at arm's length.
How many Pleiads can you count with your unaided eye? Take your time and keep looking. Most people with good vision can count 6. But with sharper eyesight, a good dark sky, and a steady gaze, you may be able to make out 8 or 9. Binoculars show several dozen.
SATURDAY, DECEMBER 17
■ Have you ever watched a Sirius-rise? Find an open view right down to the east-southeast horizon, and watch for Sirius to come up about two fists at arm's length below Orion's vertical belt. Sirius rises sometime around 8 p.m. now, depending on your location.
About 15 minutes before Sirius-rise, a lesser star comes up barely to the right of there: Beta Canis Majoris or Mirzam. Its name means “the Announcer,” and what Mirzam announces is Sirius. You’re not likely to mistake them; the second-magnitude Announcer is only a twentieth as bright as the King Star soon to make his entry.
When a star is very low it tends to twinkle slowly, and often in vivid colors. Sirius is bright enough to show these effects well, especially with binoculars.
This Week's Planet Roundup
Mercury and Venus glimmer very low in the bright afterglow of sunset. Try for them with binoculars 20 or 30 minutes after sundown, just above the southwest horizon. Venus is by far the brighter of the two at magnitude –3.9. Mercury, 5° or 6° to Venus's upper left all week, is only mag –0.6. They both get a little higher and easier day by day.
Mars is magnitude –1.7 and just past its December 7-8 opposition. It shines like a firespark low in the east-northeast in late twilight as the stars come out. It gains altitude until culminating nearly overhead around 11 or midnight. There's no missing it; Mars outshines even Sirius (which rises around 8 or 9 p.m.). Mars's fiery yellow-orange color always helps give it away. In a telescope Mars is just under 17 arcseconds wide, starting to shrink a bit.
Mars this week is between Aldebaran, lower right of it in the evening, and Capella more than twice as far to Mars's upper left. It will be pass exactly between them on December 14th. You can watch its daily progress toward and across this line more accurately by holding a yardstick or other straightedge to the sky from Aldebaran to Capella, or by stretching a string tightly from one star to the other between your hands.
Jupiter blazes white high in the south in early evening. At magnitude –2.5 it vastly outshines the stars in dim Pisces. Look for the Great Square of Pegasus above Jupiter as the stars come out, and upper right of Jupiter later in the evening. Telescopically, Jupiter is down to about 41 arcseconds wide.
Saturn, magnitude +0.8 in Capricornus, glows in the southwest as twilight fades to night. Left of it, by two fists or more, sparkles fainter Fomalhaut. To the right of Saturn by three fists or more is Altair. Catch Saturn as early as you can; as evening progresses, it moves lower and sets around 9 p.m.
Uranus, magnitude 5.7 in Aries, is high in the southeast to south during evening. It displays a tiny, very slightly blue-greenish gray disk 3.7 arcseconds wide. It a telescope at high power it's obviously non-stellar. See the Uranus finder charts in the November Sky & Telescope, page 49.
Neptune, magnitude 7.9 at the Aquarius-Pisces border, is high in early evening about 7° west of Jupiter. It's just 2.3 arcseconds wide, again non-stellar in a telescope but requiring more effort than Uranus. It's slightly bluish gray, if you have enough aperture to show color at all in something this faint. See the Neptune finder charts in the September Sky & Telescope, page 49.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.
Eastern Standard Time (EST) is Universal Time minus 5 hours. Universal Time is also called UT, UTC, GMT or Z time.
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.
You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."
Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.
"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
— Carl Sagan, 1996
"Facts are stubborn things."
— John Adams, 1770