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Northern Hemisphere gazer's guide

Dec. 1: The waning crescent moon (just two days past last quarter) passes Jupiter and Saturn within four hours shortly after midday, Eastern standard time , today. From moonrise (about midnight) until dawn this morning, the moon will be quite close to the two planets, approaching both from the right (west) while all three objects slowly climb the eastern sky. The brighter of the two objects near the moon is Jupiter, but the much brighter planet Venus rises below them and to their left before dawn.

Dec. 2: Look again this morning and you will see the moon to the east (left) of Jupiter and Saturn, between them and Venus. The moon is at apogee today, the position in its orbit where it is farthest from earth.

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Dec. 4-5: The waning moon is near Venus on these two mornings, rising about an hour before dawn while the planet Jupiter and Saturn are well above them and to their right. At dawn on the 4th, the moon is to Venus's right. On the morning of the 5th (having passed Venus about 4:00 p.m. EST on Thursday), the moon is to the left and more distant from the planet.

Dec. 8: The earliest sunset of the year in the midlatitudes of the Northern Hemisphere occurs today.

Dec. 13: The Geminid meteor shower (named for its radiant point among the stars of the constellation Gemini) reaches maximum today. After the Perseids of August, the Geminids are the most reliable and productive shower meteors of the year, though they are not usually very bright. This morning after 1 a.m. (when you will be on the "forward" side of the earth in relation to its orbital motion), you can expect to see up to 50 shower meteors per hour, less than half as many on Friday and Sunday mornings.

Dec. 19: The moon, now in waxing gibbous phase, is at perigee, where it is nearest earth.

Dec. 21: The sun arrives at the winter solstice, and winter begins in the Northern Hemisphere at 11:56 a.m., EST. At the moment, the sun is located over the Tropic of Capricorn, the most southerly position on earth where the sun appears directly overhead. The southerly position of the sun causes this to be the shortest day of the year at places north of the Equator, in the sense that the sun is above the horizon for a shorter duration than on any other day. However, it is not the date of the earliest sunset (see Dec. 8, above) or of the latest sunrise (in early January), because the standard of time that we use in describing those events is not based on the position of the sun. We use the mean sun, a fictitious celestial position determined by the average (or mean) motions of the sun.

Dec. 21: Note that the full moon occurs today. Just as the sun spends the shortest duration above the horizon in the Northern Hemisphere on this date, the full moon (when it occurs on the date of the winter solstice) is above the horizon for a greater duration than at any other time of the year. Watch how long it is from moonrise to moonset, and how high the moon is located at midnight, when it is highest in the sky.

Dec. 22: The Ursid meteor shower, radiating from a point in the constellation Ursa Major, is at maximum today, but this is not a very productive shower. You may see up to 15 shower meteors per hour in the after midnight sky this morning, but very few on the mornings before and after.

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Dec. 28-29: The last quarter moon occurs shortly after the moon rises tonight (about midnight). Jupiter and Saturn rise at just about the same time, and the moon will be just above them. Between 2 and 3 a.m. EST the moon passes the two planets in turn, Jupiter (the brighter) first, then Saturn, and rises up the sky with them, while separating slowly to their left. It will be a very attractive sky right up until dawn, at which time Venus will come along to join the morning objects above the horizon.

Dec. 30: The moon is at apogee, farthest from Earth, for the second time this month (see Dec. 2 above).

All Month: At one time or another during December, every planet but Mars is a morning star (Mercury barely makes it on the last day of the month.) As you might expect, therefore, planets are poorly represented in the evening sky this month (even the exception, Mars, is poorly placed), but the morning planets more than make up for the deficiency. And since dawn is so late in December, it isn't a great sacrifice to watch the morning stars. They put on an exciting show, with Jupiter and Saturn rising at midnight and Venus an hour before daybreak. Even Mercury is in good position for the first week or so, very low in the east at dawn, just below Venus.

Perhaps the best views of the morning objects will be about 5 a.m. or a little before, when Venus is well up in the southeast and Jupiter and Saturn are high in the south with the stars of Virgo. Best dates will be Dec. 1-4 and Dec. 28-31 when the moon, close to its last quarter phase both times, will be passing the planets in the morning sky.

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