Sky & Telescope - May 2021 - 48

MAY 2021 OBSERVING
Celestial Calendar by Bob King

A Brief
Total Lunar
Eclipse
Get ready to watch the Moon
pass through Earth's shadow.

48

M AY 2 0 2 1 * S K Y & T E L E S C O P E

more of the umbra, further contributing to the brevity of totality.
Even though the Moon appears
unusually large during the eclipse,
the difference won't be noticeable to
most observers. Indeed, the best way to
appreciate its size will be to take a photo
during the eclipse, then take another
one 6 months later during the November 19th near-total eclipse, when the
Moon's apparent diameter is just 29.8′.
Side by side, the two photos will clearly
show the difference.
As indicated on the map on page 49,
totality is visible from the western half
of North America, western South Amer-

S TOTALITY AGAIN Sky & Telescope Consulting Editor Gary Seronik captured this triptych
portrait of the most recent total lunar eclipse,
which occurred on the night of January 20-21,
2019. Like this month's eclipse, during the
2019 event the Moon passed north of the center of Earth's umbral shadow, resulting in the
northern limb being markedly brighter than the
southern edge of the lunar disk.

ica, East Asia, and Australia. Across the
continental U.S. and Canada, the Moon
will hang low in the southwest, so be
sure to find a location with an open
view in that direction to make the most
of this event.
Views improve the farther west you

Total Lunar Eclipse
May 26, 2021
North

c

Eclipti

Partial
eclipse
begins
9:45 UT

Total
eclipse
begins
11:10 UT

UMBRA
PENUMBR

South

A

Moon enters
penumbra
(unobservable)
8:46 UT

M OON PATH DIAG R A M: LE A H TISCIONE /
S&T; SOURCE: USNO

Partial
Moon
eclipse
leaves
ends
penumbra
(unobservable) 12:53 UT
13:51 UT

n's
Moo
a
p th

MidTotal
eclipse eclipse
ends 11:19 UT
11:28 UT

West

inally. After four penumbral
eclipses in 2020, on May 26th we
can enjoy the first total lunar eclipse in
more than two years. But this is a short
and sweet event.
When the Moon passes directly
through the center of Earth's umbral
shadow, totality can last up to 1¾
hours. However, during this month's
eclipse the Moon barely edges inside
the umbra, spending only 18.4 minutes
there before coasting back into the
much lighter penumbra.
The fraction of the lunar disk
immersed in the umbra is described
by the magnitude of the eclipse - the
greater the magnitude, the longer the
eclipse. A total eclipse has a magnitude of 1 or greater, and a deep eclipse
- when the Moon passes through
the center of Earth's shadow - has a
magnitude around 1.8, meaning that
the Moon lies 1.8 lunar radii inside the
umbra at mid-eclipse. With a magnitude of 1.0095, the May event is clearly
a squeaker. This also means totality will
likely be relatively bright, because the
Moon's northern limb never strays far
from the umbra-penumbra border.
May's lunar eclipse happens to coincide with a perigean full Moon, known
popularly as a " supermoon. " Perigee
occurs on May 25th at 9:53 p.m. EDT,
about 9½ hours before mid-eclipse. The
full Moon's average apparent diameter
is 31′, but during this perigean totality
it will be 33.6′ across - about 8% larger.
Ironically, the extra-large Moon fills up

East

F



Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021

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Sky & Telescope - May 2021 - Cover1
Sky & Telescope - May 2021 - Cover2
Sky & Telescope - May 2021 - 1
Sky & Telescope - May 2021 - Contents
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