Night Skies - December 2008

Venus and Jupiter continue their waltz

by Leo Enright

Locally, during December, the times of sunset vary by only about eight minutes! On the first day of the month, sunset is at 4:25PM EST, and it occurs just a few seconds earlier each evening until December 10 when it is at 4:24PM, which is the date of the EARLIEST sunset of the whole year. After the 10th, sunset is a few seconds later each evening until, by December 31 it is at 4:33PM. Meanwhile, in the mornings, sunrise is occurring a little later each day – from 7:25AM on December 1 to 7:47AM on December 31. Locally the latest sunrise of the whole year does not happen until January 3. How many of your friends realize that our date of earliest sunset is over three weeks earlier than our date of latest sunrise? Or, do they realize that neither event occurs on the date of the solstice?

For those who have an unobstructed view of the southwestern, and the southeastern, horizon, it is the time of the year to observe what is really meant by the word ‘solstice’. If you enjoy such views from your location, you should try and watch the sunset and the sunrise on as many occasions as possible during December. If done carefully, such observations will show you that the most southerly positions for the evening setting in the southwest and the morning rising in the southeast occur on December 21. The naming of the phenomenon does make sense, since the word ‘solstice’ is derived from two Latin words meaning ‘the sun’ and ‘standing still’. Since mid-summer, the sun’s setting and rising positions have been moving southward, and on this date they stop or ‘stand still’, and start to move northward again.

A second very simple observation for the month of December concerns the Full Moon. The Full Moon is always in the opposite direction in the sky from where the sun is. Often I have written about the Full Moon rising as the sun sets in the evening, or setting as the sun rises in the morning. Just as, at the time of the December solstice, the sun is furthest south of any time of the year, so also the Full Moon nearest the December solstice is the furthest north of any Full Moon during the whole year. The Full Moon this month occurs on December 12, just nine days before the solstice. In the week following Full Moon, try to observe where the moon rises and sets, and you will notice that it rises and sets, not in the east and the west, as might be expected, but in the northeast and the northwest. By coincidence, the perigee point of the moon’s monthly orbit (the point where it is closer to the earth than at any time during the month) also occurs on December 12. That means the moon that night will appear, not only further north in the sky than usual, but will also appear slightly larger and slightly brighter than usual. Over the years, photographs taken by astronomers of perigee Full Moons show them appearing very noticeably larger than apogee Full Moons. (Apogee is the point in its orbit where the moon is furthest from earth. Like the earth’s orbit around the sun, the moon’s orbit around the earth is, of course, an ellipse, not a circle.) Perigee Full Moons also bring unusually high tides for those who live along the ocean; if you have friends in the Maritimes, be prepared to hear about such occurrences in mid-December this year. Indeed, there is plenty to observe regarding the Full Moon this month.

As always on the crisp, clear nights of December, it is time to enjoy more dazzlingly bright stars in the southern sky than at any other time of the year. The large and distinctive constellation, Orion the Hunter, marches across the sky, followed by his faithful dogs, Canis Major and Canis Minor. Brilliant stars mark the shoulders, knees, and belt of Orion, and the brightest star of the whole sky, Sirius, marks the eye of his larger canine friend, while the sixth brightest star, Procyon, marks the head of his smaller companion. The famous Orion Nebula (sometimes called Orion’s Sword) can be easily seen with the unaided eye on clear, moonless nights – just below Orion’s Belt Stars. Those who have good-quality binoculars or a small telescope are able to see some of the nebula’s swirling patterns of hydrogen gas and a place in it where new stars are being born over millions of years. In fact, a very small group of those stars can be glimpsed with a small telescope.

Four of the five bright planets may be easily seen this month, with Mars being the one that is unviewable, since it is in the part of its orbit that is on the far side of the sun. Venus and Jupiter, the two brightest planets of all, continue the waltz they began two months ago. For the past 60 days we have been able to see these two brilliant objects come closer to each other – low in the southwestern sky in the hour or so after sunset. On December 1, they are at their very closest – separated by just the width of two fingers held at arm’s length. Don’t miss the spectacular view in the southwestern sky that evening, and ask your friends to join you to behold the ‘jaw-dropping’ view. Adding to the spectacle that evening will be the slender crescent moon just to the left, and up, from Venus. As the month progresses, the two planet will continue the pattern of movement they had for those 60 days, but they will appear to waltz apart now – brilliant Venus continuing to move upward and to the left, and Jupiter appearing to move slowly downward toward the horizon. In the last week of December, Jupiter acquires another planetary companion. Beginning about December 25, the planet Mercury may be seen below Jupiter, if one looks carefully beginning about 40 minutes after sunset. Binoculars may assist in finding it. Once again, a very thin crescent moon joins a pair of planets low in the southwestern sky – on December 28, the thin crescent is below Jupiter and Venus and on the 29th it is above the pair of planets. On the 30th, the slightly larger crescent is below Venus, and on New Year’s Eve, it appears just above Venus. Saturn, this month, may be seen rising in the east about midnight, for those who have a good view of the sky in that direction. It is among the stars of the constellation Leo the Lion.

Among the famous and very active meteor showers, is the one called ‘the Geminids’, which reaches its peak on December 13. In most years, at the time of its peak, numerous bright “shooting stars” could be seen throughout the night. However, this year, with the Full Moon in the sky that night, most of the meteors will be difficult to see, and only the brightest will be noticeable. Yet, it may be worth trying, and on the several nights before and after the peak. A second shower, “the Ursids”, peaks on the 22nd, and produces meteors in the northern part of the sky – also possibly worth observing.

More information about observing the spring and summer sky is available in the book entitled “The Beginner’s Observing Guide” which is now available at Sharbot Lake Pharmacy.