Sky & Telescope - January 2024 - 65

LOOKING FORWARD
This graph looks into the
future for saros 139, the cycle
that includes the April 8,
2024, eclipse. Each orange
dot in this sample represents
consecutive solar eclipses in
the series. The 2186 event
has the greatest totality since
it occurs when the Moon is
very nearly exactly at perigee
when it occurs.
Lunar Distance from Node and Perigee for Saros 139 Solar Eclipses
10°
20°
30°
40°
Gamma decreases, too,
with the Moon's nodal
distance, signaling the
steady southward shift
of the Moon's shadow
track on Earth's surface.
Gamma is +0.3431 for
the April 2024 eclipse
and will be closest to zero
(+0.0525) for the eclipse
of June 3, 2114, when
the center of the Moon's shadow passes over Saudi Arabia.
Gamma then turns negative for the subsequent southwardheading
eclipses in the series. In the last eclipse of saros 139,
on July 3, 2763 (gamma of -1.5132), the Moon's shadow
just nicks the daylight edge of Antarctica, then mostly in its
wintertime darkness.
The Babylonians were aware of the apsides of the lunar
-40°
-30°
-20°
-10°
0°
5.0°
4.0°
3.0°
2.0°
1.0°
0°
-1.0°
Distance from Ascending Node
July 16, 2186. On that date, perigee occurs within an hour
of totality. Lucky observers will get to enjoy 7 minutes and
29 seconds of totality! In fact, it's the longest eclipse in the
entire 1,262 years of saros 139. By contrast, our April eclipse
will have a respectable (but not exceptional) maximum of 4
minutes and 28 seconds.
That 2186 event gets the prize for the best eclipse in this
orbit - perigee (when it is closest to Earth) and apogee (when
it is farthest) - and when they coincided with the nodes. The
apsides have their own motion, too, completing a clockwise
revolution in 8.85 years. The Moon will be reasonably close to
its perigee for the April eclipse, having sailed past that mark
the previous evening. Being close to perigee signals a longerduration
event since the lunar disk will be larger compared to
when it's near apogee. Luckily for totality-lovers, the Moon
will be only 359,781 km from Earth when it passes in front of
the Sun on April 8th.
Compare that with the annular eclipse on October 14,
2023. It occurred half a year earlier at the opposite (descending)
node and belonged to even-numbered saros 134. During
annularity, the Moon was at a distance of 397,017 km and
close to apogee, which occurred four days earlier.
A Perfect Eclipse?
The decreasing angular separation of the Moon both from the
node and perigee with each eclipse in our saros 139 sample
suggests they will converge in a few more cycles. As a matter
of mathematical and astronomical curiosity, I plotted the
closeness of the perigee with the closeness of the nodes over
the course of a century on this saros cycle, as shown in the
graph at upper right.
As I suspected, there is indeed an upcoming saros 139
solar eclipse in which new Moon and perigee nearly coincide:
series! Sadly, I don't expect to be around to see it. All the
more reason to look forward to viewing its April saros cousin.
¢ DOUGLAS MACDOUGAL enjoys using mathematics to explore
the mysteries of early astronomy and the curious motions of
our universe. You can read his blog at douglasmacdougal.com.
-2.0°
-3.0°
-4.0°
-5.0°
SAFETY FIRST The author's grandchildren look skyward from eastern
Oregon with safe eclipse glasses during the August 21, 2017, solar
eclipse. Many readers are planning (and hoping) to witness a repeat
performance this April.
sk yand tele scope .o r g * JANUARY 2024 65
GRAPH: BEATRIZ INGLESSIS / S&T; ECLIPSE SAFETY: DOUGLAS MACDOUGAL
Distance from Perigee
1970
1988
2006
2024
2042
2060
2078
2096
2114
2132
2150
2168
2186
2204
http://www.douglasmacdougal.com http://skyandtelescope.org
Sky & Telescope - January 2024

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