2020, September 22: Equal Night Followed By Equal Light

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2020, September 14: Through a hazy sky, the moon is 5.0° to the lower left of Venus.

The sun crosses the equator at 8:31 a.m. CDT to signal a change in astronomical seasons – the Autumnal Equinox.

by Jeffrey L. Hunt

Autumn begins in the northern hemisphere on September 22, 2020 at 8:31 a.m. CDT.  The sun’s light is most directed toward the equator and for the next six months aimed at the southern hemisphere.

In the northern hemisphere, the sun is lower in the sky and daylight is shorter.

On the day of the equinox, the sun rises east and sets in the west.

The word “equinox” is taken to mean “equal night.”  Daylight and nighttime are nearly equal at 12 hours.

Venus in the morning sky.

Being a sky watcher, this writer, considers another date when daylight and darkness are equal.  In the northern hemisphere that occurs in late October.

Normally, we think of two segments of a 24-hour period, daylight and nighttime.  Day is when the sun is shining and night is when it is below the horizon.

Night, though, is made of two parts, twilight and darkness.  Twilight is that period of time – averaging about 90 minutes before sunrise and 90 minutes after sunset at the mid-latitudes – when the sky is illuminated, but it’s not dark.

When the sun is near the horizon, its possible to work outside and find your way around without artificial illumination. Crepuscular creatures wander from the tree line or fly about in the air.   

2020, July 17: The crescent moon, Brilliant Venus, and Aldebaran shine from the eastern during early morning twilight.

During mid-twilight, the brightest stars are visible. The sky is waxed with cobalt blues, golden yellows, and spectacular oranges.  A crescent moon may be visible just before sunrise or after sunset.  Venus dazzles the eye, and Mercury puts on rare performances.

Sometimes Jupiter plays tag with Venus or Mars is nearby, but the Red Planet is never at its greatest brightness when near Venus.

In the later stages of twilight, the horizon near the sun’s last rays continues to hang on to the final shreds of the sun’s glory.

Then darkness falls hard. The sky is ablaze with the night’s stars. The nocturnal animals prowl and the sky watchers gaze through their telescopes.

So, this writer looks beyond the equinox to the “equal light” days of late October when daylight and darkness are equal, at about 10.5 hours.

Take a look for the morning planets Venus and Mars before sunrise.  Jupiter and Saturn are in the south during the early evening.  Mars joins them before midnight.  Here is a summary of what’s happening with the planets during September.



Categories: Astronomy, Sky Watching

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