54 AUSTRALIAN SKY & TELESCOPE January 2018
January’s total lunar eclipse
AustraliaandNewZealandwillbetreatedtoasuperbeclipseoftheMoon.
CELESTIAL CALENDAR by S. N. Johnson-RoehrO
n the evening of Wednesday,
January 31, the first of two total
lunar eclipses for 2018 will
grace the skies above us.
Totality will be seen across eastern
Asia and the eastern Pacific. China,
Mongolia, Japan, Korea, Russia, the
Philippines and most of Indonesia will
see the eclipse in its entirety.
From New Zealand and the east
coast of Australia the event will be
visible from start to finish, with the
eclipse beginning in darkness from New
Zealand and not long after the end of
astronomical twilight from the eastern
states of Australia.
Perth will miss the opening minutes
of the event, but this is no big deal as
the subtle shadow in the early stages of a
total eclipse isn’t visible to the naked eye.
The map and diagram with timetable
on the facing page will give you an idea
of what to expect at your location.Stages of the eclipse
There are five stages of a total lunar
eclipse. The first stage begins the
moment the Moon’s leading edge slips
into the penumbra. Because the Sun
isn’t a point source, the cone-shaped
shadow cast by the Earth has two
components: the penumbra and umbra.
The penumbral shadow is about 17,000
kilometres wide and is the lighter of the
two, cast when Earth covers only part
of the Sun. We won’t see any of this
happen initially, though, as the shading
isn’t detectable until it has made its way
about halfway across the lunar disk.
Look for a darkening on the eastern side
of the Moon.
Penumbral shading becomes deeper
as the Moon moves toward the first
partial phase, which begins when the
Moon’s leading edge enters Earth’s
umbra. The umbral shadow cone
extends some 1.4 million km; at thedistance of the Moon’s orbit, the cone’s
width is about 9,000 km. When the
Moon is within Earth’s umbral cone,
no direct sunlight falls on its surface,
so the umbra is darker than the
penumbra.
The third stage of the eclipse,
totality, starts when the trailing edge
of the Moon enters the umbra. Timing
this can be iffy, as the Moon’s edge
isn’t as clearly defined as that of a
smooth sphere, but once the last peak
is in the umbra, totality begins. The
length of totality for this year’s lunar
eclipse will be 1 hour 16 minutes.
The longest duration totality can be
for a lunar eclipse is about 1 hour 45
minutes.
As the trailing edge of the Moon slips
out of the umbra, the sequence operates
in reverse, moving from totality
through a second partial and then back
into a penumbral phase.This sequence taken
during a total lunar eclipse
was captured in seven
separate exposures
with a Canon Rebel XSi
DSLR and a 20-cm f/3.3
Newtonian reflector.SEAN WALKER