From UPSC perspective, the following things are important :
Prelims level : Eclipses
Mains level : Read the attached story
India recently witnessed a partial solar eclipse. Let’s take a look over all dynamics of Eclipses.
What are Eclipses?
- An eclipse takes place when one heavenly body such as a moon or planet moves into the shadow of another heavenly body.
- There are two types of eclipses on Earth: an eclipse of the Moon and an eclipse of the Sun.
[I] A Lunar Eclipse
- The moon moves in an orbit around Earth, and 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. (This sunlight is what causes the moon to shine.)
- Instead of light hitting the moon’s surface, Earth’s shadow falls on it. This is an eclipse of the moon — a lunar eclipse.
- A lunar eclipse can occur only when the moon is full. (But not every full moon is also a lunar eclipse. Can you guess why?)
Observing a Lunar Eclipse
- A lunar eclipse can be seen from Earth at night.
- A lunar eclipse usually lasts for a few hours.
- At least two partial lunar eclipses happen every year, but total lunar eclipses are rare.
There are two types of lunar eclipses: total lunar eclipses and partial lunar eclipses.
- 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 causes Earth’s atmosphere to filter out most of the blue light. This makes the moon appear red to people on Earth.
- A partial lunar eclipse happens when only a 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 are lined up.
- In a penumbral lunar eclipse, only the more diffuse outer shadow of Earth – the penumbra – falls on the moon’s face. This third kind of lunar eclipse is much more subtle, and much more difficult to observe than either a total or partial eclipse of the moon.
[II] A Solar Eclipse
- Sometimes when the moon orbits Earth, it 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 solar eclipse.
- During a solar eclipse, the moon casts a shadow onto Earth.
There are three types of solar eclipses.
The first is a Total Solar Eclipse
- A total solar eclipse is only visible from a small area on Earth.
- The people who see the total eclipse are in the centre of the moon’s shadow when it hits Earth.
- The sky becomes very dark as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.
The second type is a Partial Solar Eclipse
- This happens when the sun, moon and Earth are not exactly lined up.
- The sun appears to have a dark shadow on only a small part of its surface.
The third type is an Annular Solar Eclipse
- An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, 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-coloured disk. This creates what looks like a ring around the moon.
- During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the Umbra. This shadow gets smaller as it reaches Earth. It is the dark centre of the moon’s shadow.
- The second shadow is called the Penumbra. The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.
Observing a Solar Eclipse
- Solar eclipses happen once every 18 months.
- Unlike lunar eclipses, solar eclipses only last for a few minutes.
Why don’t solar eclipses happen at every New Moon?
- The reason is that the Moon’s orbit tilts 5° to Earth’s orbit around the Sun.
- Astronomers call the two intersections of these paths nodes.
- Eclipses only occur when the Sun lies at one node and the Moon is at its New (for solar eclipses) or Full (for lunar eclipses) phase.
- During most (lunar) months, the Sun lies either above or below one of the nodes, and no eclipse happens.
[III] Planet Transits
- When a planet comes between Earth and the Sun, it is called a transit.
- The only 2 planets that can be seen transiting the Sun from Earth are Venus and Mercury because they are the only planets that orbit inside Earth’s orbit.
- From 2000–2199, there will be 14 transits of Mercury.
- However, Venus transits are even rarer with only 2 this century, in 2004 and 2012.
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