Night_skies_dec07

Jeff Green | Nov 30, 2006

Back toHome

TheNightSkies ofDecember, 2006:A Planetary Array and a Noted Meteor Shower

by Leo Enright

Shortened days and longer nights mark the month of December, nights that provide plenty of time to enjoy the dazzling winter constellations that dominate the southern sky in late evening twilight, and this December also provides a rare meeting of three bright planets in the morning twilight. Locally, from the beginning to the end of the month, the variation in the time of sunset is only eight minutes, and that day-to-day variation is in terms of only seconds, not minutes. On December 1, sunset is at 4:25 p.m. EST locally, with a change of only a few seconds each evening until on December 10, it is at 4:24 p.m. That is the date of our earliest sunset of the whole year. Thereafter, it is later by a few seconds each evening until it is at 4:33 on the last day of the month. Meanwhile in the mornings sunrises are occurring a bit later each day – from 7:25 a.m. on December 1 to 7:47 a.m. on the 31st. Our latest sunrise of the year does not occur until January 3. How many of your friends realize that, at this latitude, our earliest sunset is OVER 3 WEEKS EARLIER than our latest sunrise? Those people may be well aware that the date of the solstice is December 21, and they may possibly think that the term refers to the length of the day though it may actually be the shortest day, but shorter by only a few seconds than the days in the week before and after it. “Solstice”, rather, refers to the fact that “the sun stands still”, as the word means literally, and this fact can be verified if one daily observes its rising position on the eastern horizon (or its setting position on the western horizon). Those positions have been moving southward since late June, and now, on this date, they stop that movement, and begin gradually moving northward again.

Just as our sun, from this latitude, appears to be further south in the sky in December than at any other time of the year, so also the December Full Moon, being at the opposite extreme in the sky, is the northernmost Full Moon of the whole year. Therefore, on the nights of December 4 and 5 look for the Full Moon to rise not in the east, but in the northeast, and to appear almost straight overhead at midnight. When it sets in the morning, it will be not in the west, but the northwest.

This is the month to savour the bright star patterns of the southern sky where there are more very bright stars than at any other season. Foremost among them is the familiar outline of Orion the Hunter marching from left to right across the southern sky, followed by his two faithful dogs, Canis Major and Canis Minor. In a straight line to the left from the three bright stars of Orion’s Belt, we easily recognize Sirius, brightest star of the whole sky, marking the eye of the big dog. A bit further up and to the left is Procyon, sixth brightest star of all, showing the head of the smaller dog. Orion’s very bright stars indicate his shoulders, knees and belt, as they were seen by our ancestors thousands of year ago. Below the trio of ‘Belt Stars’, we have an area of the sky that is of special interest to modern astronomers. In what is called the ‘Sword of Orion’ is a vast region of hydrogen gas with many new stars currently being born – stars that are only a few million years old! This is the Orion Nebula. On a clear moonless night it can be seen with the unaided eye, but in good binoculars or a small telescope it is a truly wonderful sight. Explore this region of the sky in the late evening when Orion is well up in the southeastern sky. In another part of the sky the bright stars of the constellation Cygnus the Swan, also known as the Northern Cross, are seen in a configuration very different from that of earlier in the year because this month they appear to stand above the northwestern horizon throughout the evening hours, in the normal orientation of a Christian cross.

Among the five bright planets, only one, Venus, will be seen in the early evening this month. This brilliant planet which has not been seen at all since September may be seen only very low near the southwestern horizon and for only a short while beginning about 30 minutes after sunset. Chances of seeing it easily increase in the latter part of the month, but only those with a very good view of the horizon in that direction will see it at all. Record those days on which you see it, and for how long. In the later evening, after 10 p.m., Saturn may be easily spotted in the eastern sky among the stars of the constellation Leo the Lion. It is above and to the right from the star Regulus, brightest star in that constellation. The three other bright planets, Mercury, Mars, and Jupiter are the ones that put on a great show this month, but they delay their performance until morning twilight where all of them may be seen very low in the eastern sky. Try not to miss this “gathering of the planets”, the closest such planetary arrangement in about 32 years and something not to be repeated for about another 25 years! Though the three of them are close all month, they are closest between December 7 and December 14, and in fact, on December 10, they are appear to be almost touching each other – all three of them are within an area appearing only as big as a little fingernail held out at arm’s length! Of course, they are not really almost touching each other, but are millions of kilometres apart, and only in line with each other as seen from planet Earth. Which is which? Before December 10 Jupiter appears lower than the other two, with Mercury slightly above and Mars slightly to the right. On the morning of the 10th, Mercury also appears extremely close to a distant star in the constellation Scorpius. Please do not miss the view on that morning!! Use a pair of binoculars, if possible. Be sure to look about 30 to 60 minutes before sunrise and very low in the east-to-southeastward direction. Make certain that there are no buildings or trees blocking the view to the horizon in that direction. After the 10th and for the rest of the month, the planets will very slowly separate, with Jupiter moving upward Mars and with Mercury moving downward from Mars until by the last week of the month Mercury has disappeared completely leaving the other two planets beside the bright stars of the constellation Scorpius which is to their right.

Try also to observe the moon passing the bright planets this month. The hours before midnight on December 9 and 10 are the best times to see the waning gibbous moon move past Saturn and the bright star Regulus. In the eastern morning twilight on December 18, the very thin crescent moon just to the right of Jupiter and Mars with Mercury somewhat below – another special date to view the morning sky about 45 minutes before sunup. In the evening sky on both December 21 and December 22 the very thin crescent moon may be seen to the left of brilliant Venus, but remember that it is very low the southwestern sky 30 to 30 minutes after sundown.

Though it may not be as famous as the Perseids of August, the Geminid Meteor Shower of December is now sometimes claimed to be the year’s best, and it is certainly one not to be missed. The peak nights are December 13 and 14. Begin watching for these medium-speed “shooting stars” right after the end of twilight on both of these nights. (All of these particles travel at 35 km/sec through the atmosphere as they burn up.) Look generally in an easterly and southerly direction, and record your numbers seen per hour. Some observers may watch all night, but moonlight may interfere slightly in the latter part of the night. If those nights prove cloudy, other nights within a week of those dates may produce large numbers of these meteors also.

More information about observing the winter sky may be found in the book The Beginner’s Observing Guide now available at Sharbot Lake Pharmacy. It might be an ideal Christmas gift for a good friend. Have a blessed Christmas and a safe and happy holiday.