Sky & Telescope - April 2024 - 75

LEAH TISCIONE & GREGG DINDERMAN / S&T
sky and land get much darker, but especially
because during totality we get a
rare look at the Sun's intricate, feathery
corona: its outermost atmosphere, made
mostly of free electrons and protons
that are trapped in the Sun's loopy,
streaky magnetic field.
The partial phase of a solar eclipse
can be seen from a large swath of the
world. On April 8th that will include
nearly all of North America, Central
America, and the top of South America.
But totality is reserved for a much
smaller area. The dark center of
the Moon's shadow, the part where
the Sun is completely blocked (the
shadow's umbra) is never more than
about 260 km (160 miles) wide by the
time it reaches Earth, usually less. On
April 8th, the narrow shadow-patch of
totality will sweep across the Pacific,
Mexico, the U.S., and a bit of Canada
(see page 48). That's the part for which
so many people are traveling.
The Same Size?
By an amazing coincidence, the Sun
and Moon both appear about the
same size as viewed from Earth. That's
because the Sun, which is really about
400 times bigger than the Moon, also
happens to be about 400 times farther
away. So they both appear about the
same size in the sky (½° wide, about
the size of your little fingernail held at
arm's length).
However, their apparent sizes do vary
a bit. The Sun appears smallest in early
July every year, when Earth is farthest
away from the Sun in our orbit (at what's
called aphelion). The Sun appears largest
in early January when we're closest to it
(at perihelion). But the difference is only
3.3%, 1 part in 30.
The Moon varies by a greater amount.
It looks smallest when it's at the apogee
of its orbit around Earth, and it looks
largest at perigee. This difference can
sometimes amount to as much as 14%.
As a result, we can get two types of
" central eclipses " of the Sun. If the Moon
appears a little bigger than the Sun, the
Sun gets completely covered and the
eclipse is total. If the Sun appears a bit
bigger than the Moon, its brilliant edge
shows all around the Moon's rim and the
eclipse is annular, meaning ring-like.
The Workings in 3D
So why don't we get a solar eclipse at
every new Moon? How does the Moon
usually sneak by unseen?
The answer is that the solar system
is not perfectly flat. In particular, the
orbits of the Moon and Earth do not
lie in quite the same plane. If you drew
Earth's orbit around the Sun on a flat
sheet of paper, the Moon's orbit around
Earth would be tilted 5° out of the
paper, floating a little above and below
it except where the planes intersect.
Eclipses happen only when both the Sun
Sun & Moon Dance Steps
and Moon are close to this intersection,
along what's called the line of nodes.
A 5° tilt might not seem like much.
But it's 10 times the ½° width of the
Sun as we see it in the sky. So, when
new Moon happens somewhere other
than at the line of nodes, the Moon has
plenty of room to pass north or south of
the Sun without crossing it. That's why
solar eclipses are uncommon.
Your Eyes Are Worth
Keeping
As you've probably heard many times,
staring at the Sun's blazing surface,
even when only part of it is in view, can
damage your eyes and leave a permanent
blind spot in your vision. Never
stare at the bright Sun, whether a partial
eclipse is in progress or not.
Instead, during the partial phases
you need to look through a safe solar
filter. One example is the kind that's
made to go over the front of a telescope.
For viewing just by eye, the inexpensive
little " eclipse glasses " that we have
tested do just fine.
Or you can watch an image of the
Sun as projected onto a piece of paper
- using either a pinhole, which gives a
very small, dim, crude image, or a pair
of binoculars or a telescope, which project
a much bigger, brighter, and sharper
image onto paper. See page 72.
While the Sun is hidden during
totality, no filter is needed! ¢
First Contact
The Moon's
leading edge
appears to
touch the Sun's
limb.
(Partial Phase)
The Moon
slowly blocks
more and more
of the Sun's
face. Lasts
about an hour.
Second Contact
The Moon's
leading edge
reaches the Sun's
far edge. Totality
begins!
Third Contact
The Moon
finishes its time
completely
blocking the Sun.
Totality ends.
(Partial Phase)
The Moon
slowly slides off
the Sun's face.
Fourth Contact
The Moon's
trailing edge
slides off the
Sun.
The stages of a solar eclipse, from partial to total to partial, begin and end when the edges of the Moon and Sun appear to make contact
with each other. This happens four times from start to finish, as labeled above.
sk yand tele scope .o r g * APRIL 2024 75

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Sky & Telescope - April 2024

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