Spend a few minutes to understand why the night side of the earth faces different constellations at different times of the year. In one month the earth moves 1/12 of the way around the sun, so in that same month the stars move 30 degrees (1/12 of 360) eastward (counterclockwise) around the sun and at night the night side of the earth faces the next constellation to the east. Consequently, the constellations shift 30 degrees westward. The stars are fixed in space and the earth returns to face the same constellations after one year, but in that year Jupiter moves roughly 30 degrees eastward (counterclockwise) and Saturn 12 degrees eastward against the background of stars. Jupiter moves to the next constellation to the east each year while Saturn takes 2½ years to move to the next constellation, and Jupiter quickly leaves Saturn behind | Photo courtesy NASA/JPL-Caltech, St. George News

Stellar Vista Observatory Sky Report
John Mosley

Dec. 20 – 26

The Sky Report is presented as a public service by the Stellar Vista Observatory, a nonprofit organization based in Kanab, Utah, which provides opportunities for people to observe, appreciate, and comprehend our starry night sky. Additional information is at www.stellarvistaobservatory.org. Send questions and comments to [email protected].

Venus continues its rapid descent out of the evening sky, setting three minutes earlier each night as it moves between the Earth and Sun. This was explained in detail in earlier Sky Reports which are archived at this link. Say goodbye to the Evening Star.

Jupiter and Saturn are low in the west, to the left of Venus, and they too set earlier each night, but for a different reason. We’re not seeing their motion around the sun but the consequence of the earth’s motion. As we orbit the sun the night side of the earth faces a slightly different direction in space each day.

In a month we’ve moved 1/12 of the way around the sun and the part of the universe we face has shifted 1/12 to the left. In that time Jupiter, Saturn and the stars beyond move 1/12 of the way around the sky to the right, and set 1/12 of a day or two hours, earlier. We’ll lose Saturn next month and Jupiter in February – and then the evening sky will be devoid of bright planets and will look quite bare.

Uranus and Neptune are in the evening sky too, and if you have binoculars you can see Uranus and any small telescope will show Neptune, but the trick is to know where to look. In the olden days one would get their coordinates from an almanac and plot their positions on a star chart; nowadays one uses an app like SkySafari (my favorite) which does everything except point your telescope, and if you have a computerized telescope it will even do that.

Neptune is as far to the left of Jupiter as Saturn is to Jupiter’s right, in Aquarius, and Uranus is much farther to the left, in Aries.

Dec. 21 marks the winter solstice, the shortest day of the year (in the northern hemisphere!) and the day when the sun rises and sets as far south as it will during the year and its noontime elevation is at its lowest. For stargazers, it’s the longest night! The precise time of the solstice is 8:58 a.m. MST and that’s the first day of winter. Seasons happen because the earth is tilted on its axis; Google “winter solstice” for a complete explanation.

The Ursid Meteor Shower also peaks on Tuesday night. This is a minor shower of interest only to specialists; don’t bother.