The Sky This Week: Under the Full “Harvest” Moon on Sept. 29

By the Geoff Chester, U.S. Naval Observatory.

The Moon starts the week in the company of Saturn, then moves eastward along the ecliptic through the dim autumnal constellations.  Full Moon occurs on the 29th at 5:58 am Eastern Daylight Time.  Look for Luna just above the bright glow of Jupiter late on the evening of October 1st.  On the following night you will find her just to the south of the Pleiades star cluster.

September’s full Moon is almost universally known to Northern Hemisphere dwellers as the “Harvest Moon”.  This is not just a traditional name; it describes an astronomical phenomenon that only occurs in the autumn.

As the Moon moves around the sky, it advances about 13 degrees along its orbital path from night to night.  In the autumn, if you watch Luna’s progress from new to full phase, you will see that it passes its lowest point above the southern horizon at the time of its first quarter phase, then begins to climb northward as it waxes to full. Its orbital path intersects the eastern horizon at a shallow angle, which means that at the time of full Moon it rises at almost the same time from night to night.  Before the invention of artificial lighting, the light of the bright rising Moon added a bit of extra light after twilight, giving farmers some extra time to bring in their crops.

Here in Washington, the Moon rises about 30 minutes later on successive nights around the time of the Harvest Moon.  The effect is accentuated as you move to more northern latitudes.  In Great Britain the difference between successive moonrises is just 10 minutes, while in Stockholm the difference is 2 minutes.  Above the Arctic Circle the Moon actually rises earlier on successive nights!

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