An eclipse season
Here we break down the entire chronology of the upcoming total solar eclipse on April 8.
The usual number of eclipses in a year is four; in a few years there can be 5, 6, and even 7. The year 2024 is the usual 4-eclipse kind. What underlies this is that there are two eclipse seasons each year.
Each of them contains two (sometimes three) eclipses: one solar, one lunar, and sometimes another one of either type. The eclipse seasons are actually less than six months apart, by about 19-days, so they keep falling back that much earlier each year.
Last year they fell in April/May and October; this year they have shifted in the calendar to March/April and September/October. Next year they will have shifted back again, so that one will come in March, the second in September.
What is an eclipse season?
It is a span of about a month during which eclipses are possible at the new and full moons, because the line of the crossing point of the moon's orbit with that of the Earth's (called a "node") is pointing near enough to the sun. The moon's orbit is tilted to the Earth's at an angle of just over 5 degrees; the nodes are the two points at which the planes intersect — that is, at which the moon passes down or up through the ecliptic plane. More often than not, the new moon will either pass too high or too low to cast its shadow on the Earth. But at least twice each year, the new moon will pass close enough to a node to cover the sun, resulting in an eclipse.
One number by which eclipses can be measured is what eclipse calculators call "gamma" (γ) or the axis-offset. It is the perpendicular distance from the Earth's center to the moon's axis, or in other words, a line drawn from the sun through the center of the moon and out along the axis of its shadow. If the offset were 0, the eclipse would be exactly central; if it is more than .9972 of an Earth-radius, the middle of one body's shadow misses the other; the greater the offset, the slighter the eclipse.
For our April 8 solar eclipse, the axis-offset amounts to .3431 of an Earth radius. Therefore, the moon throws its shadow not far north of the center of the Earth (the core or shadow axis passes 1,358 miles/2,185 km from the center).
In the other dimension, that of the distance from the Earth to the moon, the moon has to be nearer than average on April 8 if it is to completely cover the disk of the sun and indeed it is: the moon arrives at perigee — its closest point to the Earth, 222,979 miles (358,850 km) — about 25 hours before new moon; thus, the disk of the moon will appear 5.66 percent larger than the sun.
The April 8, 2024 eclipse of the sun. Outside the narrow path of totality, a partial eclipse will be visible. At any location the Universal times of first contact (the beginning of partial eclipse) and last contact (the end of partial eclipse) can be interpolated from the dashed curves. Map courtesy of the United States Naval Observatory, Washington D.C. (Image credit: United States Naval Observatory)
The mechanics
A month before the eclipse (on March 10) when it came around between the sun and Earth, the moon was flying too far to the south to cast its shadow upon the Earth's surface, because it had yet to reach its ascending node which was still about 40 hours ahead. But with the April new moon the ascending node is only about 6 hours ahead; so, the shadow strikes below the Earth's south side.
The March equinox — the first day of spring in the Northern Hemisphere — comes about 20 days prior to the start of the eclipse, so the Earth has begun to tilt north pole-forward (being most so at the June solstice).
For these reasons the shadow takes a general geographic course northward and also because the moon itself is ascending northward. And as a consequence, the track of the moon's umbral shadow path across the Earth might best be described as a "lazy S," which over the course of 3 hours and 15 minutes travels approximately 9,200 miles (14,800 km) and covers over 0.52% of Earth's surface area. Sweeping such a range of latitudes from 7 degrees south to 49 degrees north, causes it to visit a great diversity of climates, landscapes and human habitats.
The story of the shadow
The penumbra — the lighter/outermost part of the Earth's shadow — meets the Earth first at 15:42:11.8 Universal Time (U.T.) over the Pacific Ocean roughly 300 miles to the east-northeast of Tahiti, the largest island of the Windward group of the Society Islands in French Polynesia. This section of our globe is at the front of the moving and rotating Earth, now just turning into the view of the sun — in other words the sunrise line. So, the crew of a ship near this point would theoretically see the sun's topmost edge, just coming above the eastern horizon, ever-so-slightly obscured by the lowest edge of the moon.
Just over an hour later, the penumbra begins to spread over the Pacific coast of Mexico, where a small dent appears on the upper right portion of the sun as the silhouette of the moon slowly encroaches upon the sun's dazzling disk.
By 18:00 UT, the bulk of the penumbra — a circle measuring roughly 5,300 miles (8,500 km) in diameter — has spread rapidly northward onto the Earth so that all of Mexico and Central America, as well as southwest Canada and much of the contiguous United States (save for the mid-Atlantic and Northeast), sees varying degrees of a partial eclipse during the late morning and midday.
According to the law of averages, a specific geographic location will experience a total solar eclipse on the order of once every 375 years. But here is a most unusual case of a city being treated to a view of two total solar eclipses within a timeframe of less than 7 years! On April 8, totality returns to Carbondale, at 1:59:14 p.m. C.D.T. and lasts 4 minutes 8 seconds.
Eclipse chronology
April 5
03:00:00 – Middle of Eclipse Season: The sun arrives at the same longitude as the moon's descending node.
April 7
17:32:00 – Moon at Perigee: Measured from the center of the Earth to the center of the moon, the distance is 222,979 miles (358,850 km).
April 8
12:18:00 – Moon's center reaches the ascending node of its orbit through the ecliptic.
15:42:11.8 – Partial Eclipse begins: First contact of moon's penumbral shadow cone with Earth at local sunrise.
16:38:49.3 – Total Eclipse begins: First contact of the moon's umbra (southern prolongation of its umbral cone) with the Earth at local sunrise.
16:39:58.5 – Central Eclipse begins: The umbra's center line first touches the Earth.
18:17:17.9 – Greatest Eclipse: The axis of the umbral shadow passes nearest (gamma = .3431 Earth radius) north of the center of the Earth. As seen from the point of greatest eclipse, the magnitude is 1.0566, that is, the moon covers the sun and .0566 of a sun-width more.
18:20:49.5 – New moon (Conjunction of moon with sun in ecliptic longitude): The moon's center is exactly north of the sun's as measured perpendicularly to the ecliptic.
18:36:00.0 – Conjunction of the moon and the sun in right ascension: The moon's center appears exactly north of the sun's as measured perpendicularly to the Earth's equator. The center of the eclipse takes place at local apparent noon, with the sun and moon on the meridian.
19:54:26.5 – Central Eclipse ends: The umbra’s center line leaves the Earth's surface.
19:55:34.2 – Total Eclipse ends: Last contact of the moon's umbra (southern prolongation of its umbral cone) with the Earth at local sunset.
20:52:18.2 – Partial Eclipse ends: Last contact of the moon's penumbral shadow cone with the Earth at local sunset.
Saros Lineage
This eclipse passes fairly near the center of the Earth (and is therefore fairly long) because it is approaching the middle of its saros series, or series of similar eclipses, at intervals of 18.03 years. This is saros series 139 and it began on May 17, 1501 with the first of 7 partial eclipses in the Arctic; it then moved south across the Earth, transitioning into no fewer than 12 of the uncommon hybrid or annular-total kind (annular at the beginning and end, but becoming total in the middle of their paths); then 43 purely total eclipses, of which this is the 11th, and the 30th eclipse overall in this series out of 71.
Those of a certain age will recall the 8th total eclipse of this series, that of March 7, 1970; a Saturday afternoon in which the umbra swept along/adjacent to the Atlantic Seaboard.
The most central member of saros 139 will be June 13, 2132, when the shadow axis passes just 73.6 miles (118.4 km) south of Earth's center. But the peak duration will continue to rise up to July 16th, 2186, which will be the longest total solar eclipse between 4000 BC and at least AD 6000 (10,000 years), lasting a maximum of 7 minutes, 29.22 seconds. Eclipse expert Jean Meeus calculates the maximum possible eclipse duration of totality in a solar eclipse is currently 7 minutes 32 seconds.
Beginning in 2619 the umbra will miss the Earth entirely, so that the series ends with 9 partials which nobody will travel to see, the last occurring on July 3, 2763 over the Southern Ocean and a small slice of Antarctica.
By Joe Rao published For Soace.com