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January 2014 Starwatch

From Deane Morrison, University of Minnesota

In January, Earth barely “laps” Jupiter in the orbital race before being lapped by Venus.

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Earth overtakes Jupiter on the 5th, when it glides directly between the planet and the sun. At that time Jupiter is at a point called opposition, when it will be opposite the sun in the sky and visible all night. Because Jupiter is traveling slightly outward in its orbit, its closest approach to Earth comes the day before opposition. Then it will be just 390 million miles from us, and this opposition will be the closest until 2020.

Jupiter comes into opposition about every 13 months. It orbits the sun in just under 12 years; thus, in a year it moves ahead a little over one-twelfth of its orbit. But after a year Earth is right back where it was, so it must move ahead a little more than one-twelfth of its orbit—slightly over a month—to catch Jupiter again. Therefore, we’ll see the next opposition on February 6, 2015.

Because Jupiter’s oppositions are separated by 13 months, some years lack them. For instance, the planet’s last opposition was December 3, 2012; there was none in 2013.

Jupiter is in Gemini, one of the bright winter constellations now well up in the east and southeast by mid-evening. The giant planet outshines everything around it, and it also stands out due to its yellowish color. After the winter stars have risen, you can compare Jupiter’s color to that of Betelgeuse, a huge red star at Orion’s northeastern shoulder, and to the white radiance of Sirius, the brightest star in the night sky, just southeast of Orion.

If you’re out admiring the winter stars, take a look at Procyon, the brightest star in Canis Minor, between Gemini and Sirius. Procyon is the eighth-brightest star in the night sky, thanks largely to being only 11.5 light-years away. Sirius, just 8.6 light-years away, also ranks as one of the sun’s neighbors. These two stars, along with Betelgeuse, are sometimes called the Winter Triangle.

But back to the orbital race. Venus laps us on the 11th; at that point it will be only about 25 million miles away and an observer on Venus would see Earth at opposition, assuming they could see through the thick clouds that envelop the planet. This event marks Venus’s official passage from the evening to the morning sky, although it will be above the horizon at both sunrise and sunset for a few days around the transition. By the end of the month, Venus establishes itself as a “morning star” in the southeast.

Also in the morning sky, Mars is brightening as Earth gains on it. Look for it high in the south at dawn, just above the bright star Spica, in Virgo. A waning moon visits Spica on the 23rd, and as it continues its drop toward the sun’s foreglow it brushes close by Saturn on the 25th.

January’s full moon arrives at 10:52 p.m. on the 15th. Algonquin Indians called this the wolf moon, for the hungry howling of the packs in the depth of winter. That night it will follow Jupiter across the sky.

Earth reaches perihelion, the closest point to the sun in its orbit, on the 4th. But at 91.4 million miles from the sun, that’s still not nearly close enough to dispel any of winter’s chill.

The University of Minnesota offers public viewings of the night sky at its Duluth campus.

Duluth, Marshall W. Alworth Planetarium: