Sky & Telescope - April 2023 - 28

Eclipse Preview
In general, sunnier weather comes with warmer temperatures.
As a result, selecting the best viewing location can be
as simple as heading south, where the warm season is most
advanced. Climatology points the determined eclipsophile to
Mexico, where spring is well underway.
Mexico
The Moon's umbral shadow first touches Earth at 16:40 UT
in the South Pacific, about 3,300 km south of the Hawaiian
Islands and 1,390 km northwest of Tahiti. Along the sunrise
terminator, the maximum duration of totality is 2m 6s as seen
from the center of the 143-km-wide path. No major landfall
occurs during the first 85 minutes of the eclipse as the
shadow sweeps northeast across the Pacific.
The umbra finally meets terra firma on Mexico's Islas
Marías at 18:05 UT (11:05 MST). This archipelago lies about
100 km off the country's west coast. Isla María Madre is
completely inside the path of totality, while Isla María Magdalena
is bisected by its southern limit. Five minutes later
(18:10 UT; 11:10 MST), the umbra reaches Mexico's Pacific
Coast in the state of Nayarit.
The central line duration of totality here is 4m 27s, and
the Sun stands 69° above the horizon. The path width has
expanded to 199 km as the shadow pursues its eastward
track, with a ground velocity of 0.70 km/second - twice the
speed of sound.
Mazatlán, a major resort city known for its beaches and
sunny weather, is 26 km northwest of the central line. Visitors
and the city's population of 500,000 will experience
4m 16s of totality. Many eclipse chasers will be attracted by
the long duration and promising weather prospects of this
destination.
After crossing a relatively narrow, 25-km-wide coastal
plain, the path climbs over the Sierra Madre Occidental
mountain range (2,600 m, or 8,500 ft elevation) and enters
the desert plains and low mountain ridges of the Mexican
Plateau (1,100 m). Durango lies about 55 km south of the
central line but still manages 3m 47s of totality.
The umbra reaches the point of greatest eclipse (the
instant when the axis of the Moon's shadow passes closest
to the center of Earth) at 18:17:20 UT (12:17:20 CST) about
150 km northeast of the city of Durango. At that moment
the Sun's altitude is 70°, the duration of totality is 4m 28s,
and the path of totality is 197.5 km wide. Although the
ground speed of the shadow is near its minimum, it's still
twice the speed of sound. The theoretical point of the greatest
duration (the maximum amount of totality) is another
95 km to the northeast but only exceeds the duration of
greatest eclipse by 0.05 seconds.
Climbing over the Sierra Madre Oriental range (1,500 m
elevation on the eclipse path), the lunar shadow descends
into the lowlands of the Gulf Coastal Plain. The city of
Piedras Negras stands on the southern side of the Rio Bravo
(the Rio Grande, as the river is called on the American side
of the border) 14 km southeast of the central line, where
totality lasts 4m 24s.
From a meteorological perspective, your best options are
along the coast near Mazatlán, inland on the high plateau
around Durango or Torreón, or toward the U.S. border near
Zaragoza. The coast and plateau have similar meteorological
histories, with monthly cloud-cover amounts averaging
around 25% to 30%. Both are subject to mid- and high-level
cloudiness associated with the subtropical jet stream; such
cloudiness is perhaps a little more frequent on the Pacific
coast. Mazatlán has to deal with the occasional morning
fog, though it should burn off before the time of the eclipse.
More troublesome on the plateau are the occasional days with
thunderstorms, which tend to form on the western mountains
and spread cloud over the eclipse track, though most of
this should come after the Moon's shadow has passed. The
MAXING OUT This panoramic photo shows a view across Mexico's high plateau north of Torreón, looking toward the central line. The image was
captured on April 7, 2022, from a location 120 km northeast of the point of maximum eclipse.
28 APRIL 2023 * SKY & TELESCOPE
JAY ANDERSON

Sky & Telescope - April 2023

Table of Contents for the Digital Edition of Sky & Telescope - April 2023