Lights on a String

Observed on February 2, 2012, approximately 6:00 pm

We’ve had a few clear nights this week in Chicago, and the mild winter gave me another opportunity for an early evening run. As I jogged around the streets of Chicago, I could not help but notice the bright string of lights stretching across the sky in the south and southwest. Just above and to the left of the fading oranges of sunset was the planet Venus, as bright as any airplane on its final approach to O’Hare. In fact, the three brightest objects in the night sky are in view right now, and they form a diagonal line starting with Venus in the southwest, sloping upward and to the left (south).

Venus, Jupiter, and the Moon

Next on the line is Jupiter, the largest planet in the solar system, and anchoring the other end of this string of lights is the waxing gibbous moon. The moon is the brightest object in the sky; Venus is the second brightest object in the sky; Jupiter is the third brightest. See for yourself how they compare to each other and how they outshine everything else. The farther I got into my run, the darker the skies got, and I could eventually pick out the star Aldebaran (the brightest star in the constellation Taurus, the bull) just below the moon.

Venus, Jupiter, and the Moon appear to be in a straight line because all of their orbits lie on planes that are nearly identical to the plane of the Earth’s orbit around the sun. This is why we can draw a picture of the Solar System (the eight planets orbiting the sun and the moon orbiting the Earth) on a flat piece of paper and have it be a mostly accurate representation. Or to use a running metaphor, we are on the Earth, in lane 3 of a track. Venus is just inside us, in lane 2, and Jupiter is a couple of lanes over, in lane 5. On the other hand, we would need a globe to accurately describe the locations of all the stars in the night sky.

The line that an object in Earth’s orbit would appear to trace around the sky is called the ecliptic. Venus, Jupiter, and the Moon never stray too far from the ecliptic. Indeed, connecting the dots between them last night was a way to approximate where this line, the ecliptic, is in the sky. The ecliptic itself is mostly stable with respect to the stars. And so, month after month, year after year, the moon and the planets visit the same stars in the sky as they progress around their orbits. Nothing in the universe is fixed, however, and even this pattern will shift over the millennia.

This was the original inspiration for the Zodiac. As the “wandering stars” (planets) traveled around the sky, they all passed through the same twelve constellations. Where they were on the day of your birth was thought to influence your destiny.
The Ecliptic

The moon orbits very quickly around the earth, essentially once a month. It is growing toward the full moon, and every day, you can see its position change as it waxes. It is moving farther and farther away from both Venus and Jupiter, and by the time you read this, it will have already left its little stellar neighbor, Aldebaran, behind. Venus orbits the sun just inside the Earth’s orbit, and thus faster than the Earth, so its position, too, noticeably changes over the weeks. Jupiter moves much more slowly because it orbits the sun much farther away then either Earth or Venus. This month, look for Venus to catch up to Jupiter. By March, they will form a brilliant pair in the southwest sky.

Next time: More about Venus

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