Astronomy

2016: Mercury’s Autumn Morning Appearance

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The speedy planet Mercury makes a  predawn appearance  in the eastern morning sky during late September. Likely  the best date to view Mercury is on September 29, 2016 when the moon appears below it as the chart shows above.  The chart above shows the pair at about 50 minutes before sunrise as seen from the Chicago area.  Find a clear eastern horizon.  At this time Mercury appears about 8 degrees above the eastern horizon, immediately above a thin crescent moon.

 The cycle begins when Mercury passes between the earth and sun (inferior conjunction) on September 13.  It rapidly rises into the morning sky.  The chart above shows the rising time of Mercury, the moon, Jupiter and Spica compared to sunrise.  Mercury rarely appears in a dark sky.  It reaches its greatest separation from the sun (greatest elongation), shown as GEW on the chart, and then descends back into bright twilight until it passes on the far side of the sun at superior conjunction on October 27.

On September 29, the moon rises at about the same time as Mercury and its view in the sky is depicted at the top of this article.

On October 11, Jupiter and Mercury rise at the same time, at the beginning of Nautical twilight, the time when the horizon can be distinguished.  This chart shows that they are less than one degree apart.  This chart is calculated for 30 minutes before sunrise when the sky is moderately bright.  Use binoculars to locate the planets.

Mercury is a difficult planet to locate.  At this greatest elongation, it is only 18 degrees west of the sun, yet the angle the plane of the solar system makes with our horizon makes Mercury easily spotted in bright twilight.  As the inner most planet, Mercury is always near the sun.  The chart above shows Mercury at greatest elongation along with the its imaginary orbit.

The September morning sky provides a view of Mercury and the reappearance of Jupiter after its Epoch Conjunction with Venus.

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