WWW.ASTRONOMY.COM 45
AS THE MOON SLOWLY
ENCROACHES on the Sun
during an eclipse, use a white-
light filter and carefully view the
edge of the Moon at 100x power
or more. You will immediately
notice its craggy profile, much
like a sawtooth. Those are the
many hills, valleys, and craters
that lie just at the brink of visibil-
ity bordering between the near
side and the far side of our satel-
lite. Watch as those lunar “teeth”
cut across the Sun’s disk, cleav-
ing through sunspot groups visi-
ble at the time. If you happen to
have access to a detailed map of
the Moon’s exact orientation at
the time of the eclipse, also try to
identify which lunar features are
silhouetted. One source of such
maps is the Virtual Moon Atlas,
which is free to download from
https://ap-i.net/avl/en/start. — P.H.Edge of the Moon
cooler central portion is called
the umbra, while the surround-
ing lighter ring is called the
penumbra. The Sun’s magnetic
field lines are perpendicular to
the surface within the umbra,
while they are angled more
obliquely within the penumbra.
Sunspots appear in rapidly
evolving groups that typically
consist of one or two larger
spots surrounded by several
smaller members. These groups
can transform within hours or
days. It’s fun to sketch or photo-
graph their appearances every
few days over a month or more.
You’ll be amazed at how much
they change in size and shape.
By tracking them for several
months, you’ll also notice that
spots at different latitudes on
the Sun move at different
speeds. The Sun’s equator
rotates once every 25 days,
while the north and south polar
regions take approximately 36
days to complete a full rotation.
You might even begin to
notice bright patches near sun-
spots. Those are called faculae,
Latin for “little torch.” Faculae
occur above the photosphere
and are best seen along the
edge, or limb, of the Sun,
where their contrast is
enhanced thanks to an effect
called limb darkening. Limb
darkening is the result of us
seeing cooler, dimmer gas in
the outer layers of the Sun as
we look along the solar limb,
instead of the hotter gas that
we see deeper inside the Sun
when we look directly at its
central region.
The Sun’s photosphere may
appear smooth at first, but up
close, we see it is comprised of
countless tiny grains called
granules. Each granule, mea-
suring a little larger than Texas,
is a convective cell of heated
plasma. As the plasma rises to
the top of the photosphere, it
cools. This causes it to sink
back into the photosphere,
where it’s reheated and recircu-
lated, like a pot of boiling
water. Because solar granules
appear so tiny from our distant
vantage point, resolving them
requires at least a 3-inch tele-
scope, 150x magnification, and
very steady seeing conditions.
And, of course, white-light
filters are ideal for viewing the
partial phases of solar eclipses.
The countdown is on for the
next Great (North) American
Eclipse on April 8, 2024, and
people are already making
plans. One thing’s for sure: Solar
filters are on everyone’s list.Hydrogen-alpha
filters and scopes
Back in the day, viewing the
Sun through a special Hα
filter was something exotic,
available only to institutions
and wealthy amateurs. Over
time, however, their prices
have dropped, and they are
now surprisingly affordable.
Brands including Coronado
(www.meade.com/solar/
solar-scopes.html), DayStar
(farpointastro.com/products/
solar-viewing) and Lunt
(luntsolarsystems.com) sellThe Sun displays a slew of stunning features when viewed through a Hɑ filter,
as seen here. JOHN CHUMACKA flare extends from the surface of the Sun on August 15, 2016, captured here
using an Hɑ filter. MICHAEL P. CALIGIURIThe author captured this white-light image of the Sun during the 2017
Great American Eclipse, revealing a striking silhouette of the Moon’s limb.
PHIL HARRINGTON