D e s e r t E x p o s u r e
December
2008
Constellation of the Month: Cassiopeia, the Seated Queen
For a number of months, we have been looking at constellations deep in our southern sky. This month, we will look at a constellation in the far northern sky, Cassiopeia, the Seated Queen. The bright first- and second-magnitude stars of Cassiopeia appear to form the letter "W," making it a very obvious constellation. Cassiopeia has three other constellation in her mythology — Cepheus, Andromeda and Perseus — whose story we've already told here.
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Click the image for a larger map.
Looking north early
on a December evening, you can see the North Star (Polaris) about halfway
up in the sky. Above it the stars of Cassiopeia form a wide "W." The
bottom of the "W" is
toward the south, so when facing north and looking two-thirds of the
way up in the sky, the "W" appears upside-down. The Milky Way
goes through Cassiopeia, which also contains many other interesting objects. |
Cassiopeia is high in our northern sky during December evenings, as it swings over the north celestial pole each day. Since it is so far north, it appears in our sky all of the time. This is called a circumpolar constellation. It is even up during the day, though it cannot be seen with the naked eye once the Sun comes up.
Only the star Schedar, at the bottom of the "W," dips below the northern horizon as Cassiopeia swings underneath Polaris. Schedar disappears once each day — or, more accurately, every 23 hours, 56 minutes and 4.099 seconds. This is called a sidereal (star) day, since any star in the sky will be back in the same place in the sky after this amount of time has passed. It is the true mechanical rotation period of the Earth. Our clocks, however, record a day being a full 24 hours — a solar day, the period of time it takes the Sun to return to the same point in the sky.
So the Sun and the surrounding stars today will not make it back together to the same place in the sky at the same time tomorrow. The stars will arrive first. Four minutes later, the Sun will be back in place, but the stars will have moved four minutes' travel farther west, almost one degree of arc. Each day moves the Sun eastward by just under a degree, so after 365 days, the Sun will be back among the stars accompanying it today.
The Sun is not really moving around the sky. The motion of the Earth as it orbits the Sun makes the Sun appear to move against the starry background. Each day the Earth moves just under a degree around the Sun, making the Sun appear to move against the background stars.
Each hour on our clock is defined as 1/24th of a solar day. The difference between the sidereal and solar day is the angle the Earth moves around the Sun during the time it takes the Earth to rotate once (one day). But the Earth does not move at a constant rate around the Sun. It is in a slightly elliptical orbit, so it moves slowest when farthest from the Sun (aphelion) and fastest when closest to the Sun (perihelion). The Earth's rotation is not affected by the distance to the Sun, so some days the Sun travels a little farther among the background stars than other days.
The Earth is closest to the Sun in early January, when the Sun appears to be moving eastward the fastest, and farthest in early July, when the Sun's eastward progress is slowest. If the Earth did not speed up and slow down in its orbit, the Sun would pass due south of us at noon local time every day; instead, this happens only four times a year. In mid-February, the Sun runs a maximum 14 minutes "late" as it travels more quickly eastward due to the January perihelion. In early November, the Sun is 16 minutes "early" (west) of noon local time due to the July aphelion.
The Planets for December 2008
On the evening of Dec. 1, take a look in our southwestern sky. The two brilliant "stars" you will see there are the two brightest planets in our sky, only 2 degrees apart. For the past few months, Venus has been moving eastward away from the Sun, getting higher in the west every day. Jupiter is also moving eastward, but at a much slower rate. Finally, on the first day of this month, Venus catches up with Jupiter and then passes it. This is a spectacular conjunction, and the next night the crescent Moon will join in the fun.
Watch the Skies (all times MST) Dec. 1, 9 a.m. — Moon 0.8 degrees north of Venus Dec. 5, 2:26 p.m. — First Quarter Moon Dec. 12, 9:37 a.m. — Full Moon Dec. 18, 8 p.m. — Moon 6 degrees south of Saturn Dec. 19, 3:29 a.m. — Last Quarter Moon Dec. 21, 5:04 a.m. — Winter Solstice Dec. 27, 5:22 a.m. — New Moon Dec. 28, 9 p.m. — Moon 0.7 degree north of Mercury Dec. 29, 2 a.m. — Moon 0.6 degree south of Jupiter Dec. 30, 10 p.m. — Jupiter 1.3 degrees north of Mercury |
Venus starts the month in Sagittarius; it scoots through Capricornus and into Aquarius just before the month ends. Venus is still on the far side of its orbit, so it is gibbous, but waning. On Dec. 1, Venus is 69 percent illuminated and 16.7 seconds-of-arc across at magnitude -4.1. By the end of the month, it will be 57 percent illuminated and grown to 21.4 seconds-of-arc across at magnitude -4.3. Venus sets around 8:15 p.m.
Jupiter is slipping into the evening twilight, and by the end of the year, it will be too close to the Sun to see. With its low altitude, it will not be in a good position for telescopic observing. Even so, at mid-month Jupiter is 33.1 seconds-of-arc across and glows at magnitude -2.0. The King of the Planets will be in Sagittarius all month.
The Ringed Planet comes up just before midnight in the constellation Leo. Its yellow glow shines at magnitude +1.0 as it slowly brightens. Saturn's disc is 17.9 seconds-of-arc across while the Rings are 40.6 seconds-of-arc across. The rings are now tilted up only 0.8 degrees with their southern face showing. Next year we will see them (or, more accurately, not see them) edge-on.
Mercury is in our evening sky, but just barely, as December comes to an end. On Dec. 30, you can find Mercury just over a degree south of Jupiter. Look very low in the west-southwest at 6 p.m. with binoculars and you may be able to find both. On New Year's Eve, Mercury will be magnitude -0.7 and 6.2 seconds-of-arc across, 71 percent illuminated, in Sagittarius as it will have been the last three weeks of the month.
Astronomical winter begins with the Winter Solstice on Sunday, Dec. 21 at 5:04 a.m. If you are going to look for Jupiter and Mercury on New Year's Eve, hold off on starting your celebrating until after you are back inside. May you have happy holidays and a great New Year and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.
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Watching the skies: Jupiter and Venus
(bottom) join the crescent moon above the continental divide on December
1. (Photo by Scott van Linge) |