Eclipses

Solar eclipse

• Every planetary body casts a shadow. This shadow becomes evident when it falls on another planetary body. When we find ourselves in the shadow of the Moon, we experience the so-called solar eclipse. A shadow cast by the Earth on the Moon is called a lunar eclipse.
• Like all planetary shadows, the moon's shadow is made up of two parts called the umbra and penumbra. The umbra is the conical portion of the shadow which excludes all light from from the Sun. The penumbra is that part of the shadow which surrounds the umbra and allows part of the sun's light to be visible. This short video describes the various shadows.
• A total solar eclipse, or totality, occurs when the Moon's umbra reaches the surface of the Earth during the day. Because the umbra never covers more than a relatively small section of the Earth's surface, totality at a given location on Earth is considered a rare event.
• For those observers who find themselves in the umbra, the Sun's ghostly outer atmosphere, or corona, becomes visible as an irregularly shaped halo surrounding the blotted-out disk of the Sun. In addition, during the seconds or minutes of totality, the sky is sufficiently dark that stars and planets become visible. Mercury is perhaps more commonly seen at total eclipses than at other times, when it is never far from the horizon at sunset or sunrise.
• A partial solar eclipse occurs when only a portion of the Sun's face is covered because the observer is in the penumbra.
• An annular eclipse is a special type of partial solar eclipse in which only an outer ring ("annulus") remains unblocked. In this case, the Moon is at or near apogee (far point in its orbit), and so is too small to cover the Sun. No corona is seen in this case because even the small amount of the Sun still visible completely overwhelms the corona's faint glow.
• Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, it has been estimated that they recur at any given place only once every 370 years, on average. One of the reasons that total solar eclipses are rare is that although the penumbra is some 7000 kilometers across, the umbra is always very small. Even under the most favorable circumstances, its diameter never exceeds 270 kilometers.
• Because the shadow sweeps across Earth's surface at speeds of 1700-8,000 km/h (depending on latitude), the total eclipse lasts no more than a few minutes at a given location, with a maximum duration of about 7.5 minutes. As the moon begins to uncover the surface of the sun, the eclipse proceeds through partial phases for approximately an hour until the sun is once again completely uncovered.
• Eclipse darkness was famously used in 1919 and 1922 to test Einstein's general theory of relativity, which predicted that the sun's mass would apparently bend light (by warping space) enough to displace slightly the positions of stars near the sun. During the 1919 eclipse, the sun fortuitously was located in a cluster of stars during totality. When the results became known, Einstein immediately gained his lasting fame with scientists and with the public. His prediction has been better verified by other means in recent years, though, so eclipses are no longer used for testing relativity.
• This site shows a list of eclipses at various times all over the world. The next total solar eclipse over North America visits Mexico, the United States, and Canada on April 8, 2024.

Lunar eclipse

• When the Sun and the Moon are in exactly opposite directions, as seen from Earth, Earth's shadow sweeps across the Moon, temporarily blocking the Sun's light and darkening the Moon. This phenomenon is known as a lunar eclipse.
• Unlike the case for a solar eclipse, a lunar eclipse is simultaneously visible from all locations on Earth's night side, not just in a narrow swath.
• From Earth, we see the curved edge of Earth's shadow begin to cut across the face of the full Moon and slowly eat its way into the lunar disk. Usually, the alignment of the Sun, Earth, and Moon is imperfect, so the shadow never completely covers the Moon. Such an occurrence is known as a partial lunar eclipse.
• Occasionally, the entire lunar surface is obscured in a total lunar eclipse. Total lunar eclipses last only as long as is needed for the Moon to pass through Earth's shadow—no more than about 100 minutes. During that time, the Moon often acquires an eerie, deep red coloration—the result of a small amount of sunlight that is refracted (bent) by Earth's atmosphere onto the lunar surface, preventing the shadow from being completely black.
• While the penumbra is often sketched as uniform, in reality the penumbra shades gradually from the completely dark umbra out towards the edges. The reason is simple: as you move outwards away from the edge of the umbra, you will see an increasing fraction of the Sun peeking out from behind the Moon, as shown in this very nice Mir image of the 1999 Aug 11 eclipse shadow.

Line of Nodes

• An eclipse occurs when Earth, Moon, and Sun are precisely aligned. If the Moon's orbital plane lay in exactly the plane of the ecliptic, this alignment would occur roughly twice a month. However, the Moon's orbit is inclined at about 5° to the ecliptic, so not all configurations are actually favorable for producing an eclipse.
• For an eclipse to occur, the line of intersection of the two planes must lie along the Earth-Sun line. Thus, eclipses can occur only at specific times of the year. The two points on the Moon's orbit where it crosses the ecliptic plane are known as the nodes of the orbit. The line joining them, which is also the line of intersection of Earth's and the Moon's orbital planes, is known as the line of nodes. This video tutorial may help with the visualization of the different eclipses and line of nodes.
• The Sun, Earth and nodes are aligned twice a year, and eclipses can occur during a period of about two months around these times. There can be from four to seven partial eclipses in a calendar year, which repeat according to various eclipse cycles, such as the Saros cycle.
• Times when the line of nodes is not directed toward the Sun are unfavorable for eclipses. However, when the line of nodes briefly lies along Earth-Sun line, eclipses are possible. These two periods, known as eclipse seasons, are the only times at which an eclipse can occur.
• Notice that an eclipse season does not guarantee that an eclipse will occur. For a solar eclipse, we must have a new Moon during an eclipse season. Similarly, a lunar eclipse can occur only at full Moon during an eclipse season. In practice, 2 lunar eclipses typically occur in most calendar years. However, there can be up to 3 in a given year, or none at all. Solar eclipses occur 2-5 times per year, although totality occurs only once every 18 months on average.

Saros cycle

• The gravitational tug of the Sun causes the Moon's orbital orientation, and hence the line of nodes, to change slowly with time. The result is that the eclipse seasons gradually progress backward through the calendar, occurring about 20 days earlier each year and taking 18.6 years to make one complete circuit. This phenomenon is known as the regression of the line of nodes and the cycle is called the saros cycle.
• The regression of the line of nodes of the Moon's orbit causes Earth's rotation axis to wobble slightly, changing the angle between Earth's axis and the ecliptic by plus or minus 9 arcseconds every 18.6 years. This additional motion, which is superimposed on Earth's precession, is known as nutation.

Frequency of eclipses

• The number of solar eclipses (partial, annular or total) is 2-5 per year. Total solar eclipses occur, on the average, about once every 18 months somewhere on Earth and recur in the same place roughly once every 360 years.
• At any given location, up to 3 lunar eclipses can occur per per year, but some years there may be none.
• Because we know the orbits of Earth and the Moon to great accuracy, we can predict eclipses far into the future.
• The maximum number of eclipses (solar and lunar) that can occur in a year is 7, but typically fewer occur.
• The last year with a total of seven eclipses in it was 1982. The next time 7 eclipses will occur in one year is 2038 (4 penumbral lunar eclipses and 3 solar eclipses).