Sky and Telescope - September 2015 - 45

OBSERVING

Fred Schaaf welcomes your
comments at fschaaf@aol.com.

Northern Hemisphere's Sky

Fred Schaaf

Stars for a Moonlit Night
A total lunar eclipse brings out a sky full of stars.
very near full phase. Last quarter Moon is even dimmer
than ﬁrst quarter due to its higher proportion of the dark
maria. The lunar light is half as bright as full Moon not
7 days before full (around ﬁrst quarter) but a mere 2½
days before. This explains why observations of the stars
and even extended deep-sky objects through telescopes
are not as compromised by most phases of the Moon as
people expect.
A lesson can be learned from meteor observers. They
simply look for meteors in a direction away from the
Moon or, if the Moon is especially bright, may block the
direct moonlight with a tree, building . . . or even an
umbrella. When it's raining moonlight (the grand Mare
Imbrium is "the Sea of Rains"), use an umbrella or parasol - or would it be called a paralune?
The starry second nightfall of lunar eclipse.
Weather permitting, the dimming of the Moon on the
American evening of Sunday, September 27th, will be
the 22nd total eclipse of the Moon I've seen in my life.
Among all its beauties, one of my favorites will be the
awesome fall of a second, much deeper nightfall that
occurs during it. Of course, you'll only experience this
deeper night within night if you are at a site with minimal humanmade light pollution. ✦

CANSTOCKPHOTO.COM

This column is normally dedicated to the stars. But
the stars do cohabit the night sky with other celestial
objects. And one such nighttime object radically aﬀects
the visibility of stars. It is, of course, the Moon.
Star-people vs. Moon-people. It sounds like a conﬂict in a science-ﬁction movie - but what I'm asking is
how you see yourself: as a star-person or a moon-person?
Do you enjoy exploring all the details of the brightest
satellite in the solar system? Or do you like to travel and
revel in the starry universe?
Most of us, of course, like both lunar and stellar
observing. But September can be a frustrating month for
would-be star (and deep-sky) observers due to the extra
hours of bright moonlight cast into night by the Harvest Moon. I've written before about the Harvest Moon
versus the late-summer Milky Way. But this year we have
a special solution that opens to stargazers a wonderful
window on the starry universe right in the midst of the
night of the full Harvest Moon. The solution is a total
lunar eclipse.
The magic - and manageability - of moonlight.
Before we talk about this month's eclipse and its eﬀect
on the stars, however, let's get some things straight
about moonlight.
First of all, we should never begrudge it. It's just too
magical - even scientiﬁcally speaking. Moonlight covers everything like the ﬁnest of meshes. But consider
some of the scientiﬁc facts about this seemingly mystical
radiance. White paper in moonlight is about 2,000 times
darker than black velvet in sunlight. On the other hand,
the light of the full Moon is about 33,000 times brighter
than Sirius. It is more than 2,000 times brighter than
Venus when the planet is brightest (which Venus is, in
this month's pre-dawn sky). Full moonlight is even about
1,000 times brighter than the combined light of all the
nighttime stars. And it isn't millions of times dimmer
than the Sun, just 400,000 times dimmer.
But there's good news for those of us who want to do
some stellar observation while the Moon is in the sky.
The ﬁrst quarter Moon - a half-lit Moon - isn't half
as bright, but only 1/11th as bright as the full Moon. This
is due to the terminator and the many large shadows
near it stretching down the Moon's central meridian at
ﬁrst quarter, as well as to the lack of a special kind of
retro-reﬂecting that occurs from the lunar surface only

Sk yandTelescope.com September 2015

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Sky and Telescope - September 2015

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