Total Eclipse of the Moon
Sunday evening, September 27
Residents of Borrego Springs and surrounding communities – and the entire United States – will be
treated to a total eclipse of the moon on the evening of Sunday, September 27. This is the last total
eclipse of the moon visible locally for almost 2½ years.
An eclipse of the moon happens when the moon moves into the shadow of the earth and grows dark.
This can happen only at full moon, when the moon is opposite the sun in the sky. Usually the moon’s
orbit takes the moon above or below the earth’s shadow and in most months there is no eclipse, but if
the alignment is just right the moon can move into and through the shadow, and for an hour or so the
moon grows dark as sunlight is blocked by the earth.
The visible phase of the eclipse begins at 6:07 p.m. PDT when the moon begins to move into the
inner (umbral) part of the earth’s shadow – but as seen from Borrego Springs the moon will not rise
until about 6:40 p.m. For best viewing find a place with a low eastern horizon (or be prepared to wait
until the moon rises from your location).
At moonrise the moon has moved about a third of the way into the earth’s shadow and the moon’s left
third is noticeably darker than the rest. During the next 40 minutes the moon moves deeper into the
earth’s shadow and the shadow creeps across the moon’s face, moving from left to right. At 7:11 p.m.
the moon moves fully into the earth’s shadow and totality begins. At that moment the moon is a scant
7 degrees above the true horizon.
The eclipse is total from 7:11 p.m. until 8:23 p.m. with mid-eclipse at 7:47. When totality ends at 8:23
the moon is 22 degrees high (1/4 of the way up the sky) and a bit south of due east. The eclipse is
partial for the next hour as the moon slowly moves out of the earth’s shadow, and the eclipse ends at
9:27 p.m.
Assuming that the moon is above your local horizon, the best time to look is just before 8 p.m. when
the moon is deepest in the earth’s shadow.
Note that the sun sets as the moon rises, so the sky is quite bright at moonrise. The brightness of the
early evening sky may make it hard to discern the partially eclipsed moon.
So step outside, face east, and enjoy the free show the evening of September 27. Binoculars will
enhance the view. While watching the moon, pause to notice the planet Saturn very low in the
southwest. Saturn is the sole planet in the evening sky, and it sets at 10 p.m. Saturn is in Scorpius,
whose stars stretch to the lower left of the planet. The bright stars of the Summer Triangle – Vega,
Deneb, and Altair – are high overhead.
The moon darkens when in the earth’s shadow, but it doesn’t disappear. Sunlight refracted around
the edge of the earth and through the earth’s thin atmosphere, falls on the moon, and this refracted
sunlight is reddened for the same reason that sunsets are often red – red wavelengths of light
penetrate the atmosphere better than other wavelengths. The moon usually takes on an orange or
red color that is caused by light from all the sunsets and all the sunrises of earth!
If you were on the moon at this time, you would see
the earth move in front of the sun in a total eclipse of
the sun. Perhaps future astronauts will enjoy such
sights from their lunar bases.
The eclipse is visible simultaneously from all of North
America and Europe, although the farther east you
are, the later the eclipse happens (because of time
zone changes) and the higher the moon appears in
the sky. Adjust for the local time zone if necessary.
The next eclipses of the moon visible from Borrego
Springs are a total eclipse on the morning of January
31, 2018, and another total eclipse on the evening of
January 20, 2019.