Originally published in the Gettysburg Times, 9/21/15
Yes, readers, that is really a term you may hear applied to
the full moon and lunar eclipse of September 27-28, 2015. Let’s break it down.
Super: at its closest point in its orbit on the 28th,
the moon will appear a little larger than average. Some people even call it a
“supermoon.” For some perspective,
however, note that an average full moon is the same apparent size as a quarter
held 103 inches away from you. If the quarter were 98 inches away instead, that
would be a “supermoon.” When looking for a big moon, the “moon illusion” may
make a bigger impression. It is a poorly understood optical illusion that
causes us to overestimate the size of sky objects near the horizon.
Harvest: “harvest moon” is a name out of folklore given to
the full moon nearest the autumnal equinox. This year the equinox, and
therefore the first day of fall, occurs on September 23.
Blood: “blood moon” is a grotesque and rather inaccurate
name for a total eclipse of the moon. The moon, when it’s in the shadow of the
earth, can range in color from light orange to coppery to iron-gray. You might
be asking why, if the moon is in the earth’s shadow, it’s getting any light at
all? That’s a good question – look for the answer later in the column.
Here’s what you need to know to observe the lunar eclipse.
Look for the moon in the southeast as it gets dark on the night of the 27th.
Between 8:30 and 9:00 you may see the edge of the moon darkening during the
penumbral phase. Then at 9:07 the dark umbra of the earth’s shadow will begin
to creep across the face of the moon. After a while, you’ll see that the edge
of the shadow is curved – one of the non-astronaut’s best looks at the
curvature of the earth. At 10:10, the moon will be completely covered. You will
still be able to see it, though, as mentioned above. The light that reaches the
moon during an eclipse is refracting through the earth’s atmosphere, where it
gets colored just as our sunrises and sunsets do. If the earth had no atmosphere,
the edge of its shadow would be sharp and the eclipsed moon would be invisible,
but then we would not be here to observe the event. At 11:24 the total phase
will end, and at 27 minutes after midnight the umbra will have left the moon.
The next total eclipse of the moon visible from Gettysburg won’t be until
January 2019, so let’s hope the weather is good.
The eclipse isn’t the only show. Keep
an eye on the morning sky. Venus is now shining brightly in the east before
sunrise. Jupiter is below it and will be climbing and closing the gap over the
next few weeks. The waning crescent moon will be near Venus on the morning of
October 8 and near Jupiter the next morning, for a good photo opportunity. The
planet Mars and the star Regulus are both nearby, but they are fainter and more
difficult to pick out in the predawn light.
As it climbs, Jupiter will be less than a degree from Mars on October 17
and very close to Venus at the end of the month. Look for more on that conjunction
in the next column.
