Ever heard of eclipses? I am sure you must have. Ever seen one? If you have then you are very lucky, and if you have observed a total or annular solar eclipse you are even luckier and I am jealous. The word eclipse is derived from the ancient Greek noun ἔκλειψις (ékleipsis), which means “the abandonment”, “the downfall”, or “the darkening of a heavenly body. 

What is an Eclipse?

For any two objects in space, a line can be extended from the first through the second. The latter object will block some amount of light being emitted by the former, creating a region of shadow around the axis of the line. Typically these objects are moving with respect to each other and their surroundings, so the resulting shadow will sweep through a region of space, only passing through any particular location in the region for a fixed interval of time. As viewed from such a location, this shadowing event is known as an eclipse.

Typically the cross-section of the objects involved in an astronomical eclipse are roughly disk shaped. The region of an object’s shadow during an eclipse is divided into three parts:

• The umbra, within which the object completely covers the light source. For the Sun, this light source is the photosphere.
• The antumbra, extending beyond the tip of the umbra, within which the object is completely in front of the light source but too small to completely cover it.
• The penumbra, within which the object is only partially in front of the light source.

Eclipses on Earth

On earth lunar eclipses and solar eclipses are the major form of eclipses which occur here on Earth.

Lunar eclipse:

The Moon moves in an orbit around Earth. At the same time, Earth orbits the Sun. Sometimes Earth moves between the Sun and the Moon. When this happens, Earth blocks the sunlight that normally is reflected by the Moon. Instead of light hitting the Moon’s surface, Earth’s shadow falls on the Moon. This is an eclipse of the Moon, or a lunar eclipse. A lunar eclipse can occur only when the Moon is full. A lunar eclipse usually lasts for a few hours. At least two partial lunar eclipses happen every year, but total lunar eclipses are rare. It is safe to look at a lunar eclipse. A lunar eclipse can be seen from Earth at night. 

There are two types of lunar eclipses:

1. Total lunar eclipse- A total lunar eclipse occurs when the Moon and the Sun are on exact opposite sides of Earth. Although the Moon is in Earth’s shadow, some sunlight reaches the Moon. The sunlight passes through Earth’s atmosphere, which filters out most of the blue light. This makes the Moon appear red to people on Earth.
2. Partial lunar eclipse-A partial lunar eclipse happens when part of the Moon enters Earth’s shadow. In a partial eclipse, Earth’s shadow appears very dark on the side of the Moon facing Earth. What people see from Earth during a partial lunar eclipse depends on how the Sun, Earth and Moon align.

Solar Eclipse:

Sometimes when the Moon orbits Earth, the Moon moves between the Sun and Earth. When this happens, the Moon blocks the light of the Sun from reaching Earth. This causes an eclipse of the Sun, or a solar eclipse. During a solar eclipse, the Moon casts a shadow onto Earth. Solar eclipses happen every 18 months somewhere on Earth. Unlike lunar eclipses, solar eclipses last only a few minutes.

There are three main types of solar eclipses:

1. Total solar eclipse: A total solar eclipse is visible from a small area on Earth. The people who see the total eclipse are in the center of the Moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to occur, the Sun, Moon and Earth must be in a direct line.
2. Partial solar eclipse: This happens when the Sun, Moon and Earth are not exactly aligned. The Sun appears to have a dark shadow on a small part of its surface.
3. Annular solar eclipse: An annular eclipse happens when the Moon is farthest from Earth. Because the Moon is farther away, it seems smaller. It does not block the entire view of the Sun. The Moon in front of the Sun looks like a dark disk on top of a larger Sun-colored disk. This creates what looks like a ring around the Moon.

Eclipses on other planets

The gas giant planets have many moons and thus frequently display eclipses. The most striking involve Jupiter, which has four large moons and a low axial tilt, making eclipses more frequent as these bodies pass through the shadow of the larger planet. Transits occur with equal frequency. It is common to see the larger moons casting circular shadows upon Jupiter’s cloud tops.

On the other three gas giants (Saturn, Uranus and Neptune) eclipses only occur at certain periods during the planet’s orbit, due to their higher inclination between the orbits of the moon and the orbital plane of the planet. The moon Titan, for example, has an orbital plane tilted about 1.6° to Saturn’s equatorial plane. But Saturn has an axial tilt of nearly 27°. The orbital plane of Titan only crosses the line of sight to the Sun at two points along Saturn’s orbit. As the orbital period of Saturn is 29.7 years, an eclipse is only possible about every 15 years.

On Mars, only partial solar eclipses (transits) are possible, because neither of its moons is large enough, at their respective orbital radii, to cover the Sun’s disc as seen from the surface of the planet. Eclipses of the moons by Mars are not only possible, but commonplace, with hundreds occurring each Earth year. There are also rare occasions when Deimos is eclipsed by Phobos. Martian eclipses have been photographed from both the surface of Mars and from orbit.

Pluto, with its proportionately largest moon Charon, is also the site of many eclipses. A series of such mutual eclipses occurred between 1985 and 1990. These daily events led to the first accurate measurements of the physical parameters of both objects.

Eclipses in 2021

• May 26, 2021 — Total Lunar Eclipse
• Jun 10, 2021 – Annular Solar Eclipse
• Nov 18–19, 2021 — Partial Lunar Eclipse
• Dec 4, 2021 – Total Solar Eclipse

Scientists use solar eclipses as an opportunity to study the Sun’s corona. The corona is the Sun’s top layer. During an annular eclipse, NASA uses ground and space instruments to view the corona when the Moon blocks the Sun’s glare.

https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-an-eclipse-58/ http://en.wikipedia.org/wiki/Eclipse