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Moon and Saturn

May 22, 2013

The planet Saturn perches to the left of the Moon as night falls this evening. It looks like a bright golden star. The true star Spica stands to their upper right.

Spica is the brightest star of Virgo. And Saturn has been inside that constellation’s borders for most of the last four years.

One reason is that Virgo is one of the largest constellations. Another is that Saturn is so far away from the Sun that it takes the planet almost 30 years to complete a single orbit. So as seen from Earth, it takes that same amount of time for Saturn to complete one circle against the background of stars.

That “circle” isn’t smooth, though. Instead, for a few months each year, Saturn reverses its normal eastward motion and heads westward.

That’s because each year or so, Earth catches up to Saturn and passes it by. As that happens, our viewing angle to the planet changes. It’s like passing another runner on a jogging trail. When you’re behind the other runner, she appears to move forward compared to objects in the distance, such as buildings or mountains. As you catch up and pass her by, though, she briefly appears to move backward against that same background — a result of your changing viewing angle.

Saturn actually moved into the adjoining constellation Libra for a few months, but its reverse motion brought it back into Virgo. It’ll remain there for about three more months, when it’ll once again leave Virgo behind — not to return until the year 2038.


Script by Damond Benningfield, Copyright 2013

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