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What Are Solar Eclipses?

Next Annular Solar Eclipse: Thu, Sep 1, 2016 … See animation

Solar eclipses happen when the Moon moves between Sun and Earth, blocking the Sun's rays and casting a shadow on Earth.

Illustration image

A Total Solar Eclipse (Ill. not to scale)

Total Solar Eclipse: The Moon's umbra blocks the sun's rays from reaching a small area on Earth, while the penumbra causes a Partial Solar Eclipse in a larger area. (Not to scale)

Solar eclipses can take place only during a new Moon.

The Moon's Shadow

If you want to see a solar eclipse, you must be in the path of the Moon's shadow, which has 3 distinct parts:

  • Umbra: The innermost and darkest part of the Moon's shadow. The Sun's light is blocked in places on Earth where the umbra falls. The Sun's disc is not visible anymore.
  • Penumbra: The outermost and the lightest part of the Moon's shadow. Only part of the Sun's light is blocked in places on Earth where the Moon's penumbra falls. The Sun's disc is partly visible.
  • Antumbra: The Moon's antumbra lies beyond the umbra. It appears with the growing distance from the Moon. From Earth, the Moon appears smaller and cannot completely block the Sun, so the Sun's outer rim is still seen.

Types of Solar Eclipse

There are 4 types of solar eclipses and they are determined by what part of the Moon's shadow falls on the Earth:

  • Total: A total solar eclipse takes place when the Moon completely covers the Sun and casts its umbra and penumbra on Earth. A total eclipse of the Sun can only take place when the Moon is at perigee. You can experience a total solar eclipse if you're in the path of the Moon's umbra. You can see a partial eclipse at a place where the Sun's penumbra falls.
  • Partial: Partial solar eclipses happen when the Moon does not completely cover the Sun's disc and casts only its penumbra on Earth.
  • Annular: Annular solar eclipses occur when the Moon's antumbra falls on Earth. The Moon's disc covers the center of the Sun's disc, leaving the Sun's outer edges uncovered. An annular eclipse of the Sun can only take place when the Moon is at apogee.
  • Hybrid: Hybrid eclipses are rare. They happen when an annular eclipse turns into a total solar eclipse.

Only at New Moon

For a solar eclipse to take place, the Sun, Moon and Earth must be aligned in a perfect or near perfect straight line – an alignment that astronomers call syzygy. This can only happen during the new Moon phase.

Even though a new Moon is necessary for a solar eclipse to take place, eclipses of the Sun do not happen every new Moon night. This is because the plane of the Moon's orbital path around the Earth is inclined at an angle of 5° to the Earth's orbital plane around the Sun (ecliptic). The points where the 2 orbital planes meet are called lunar nodes. Solar eclipses occur only when a new Moon takes place near a lunar node.

Eclipse Season

Illustration image
©bigstockphoto.com/Marbo Stock

The Sun must also be close to the node so it can form a perfect or almost perfect line with the Moon and Earth. This alignment happens twice a year and usually lasts on average for 34.5 days. This period is called an eclipse season. At least 2, or 3 eclipses at the most, occur during every eclipse season.

A synodic month is the period from one new Moon to the next. It is roughly 29 days long and is shorter than an eclipse season. Therefore, there will be at least one new Moon and a solar eclipse, and one full Moon and a lunar eclipse during an eclipse season. This means that solar eclipses and lunar eclipses tend to come in pairs – a solar eclipse always takes place about two weeks before or after a lunar eclipse.

More Solar Eclipses Than Lunar

If one ignores the very hard to detect penumbral lunar eclipses, solar eclipses outnumber lunar eclipses almost 3 to 2. On average, a century sees about 240 solar eclipses and about 150 lunar eclipses.

Despite this, for most people a solar eclipse is a much rarer sight than a lunar eclipse. There are two reasons for this paradox:

  • We are on the body casting the shadow during a lunar eclipse, so everyone on the night-side of the Earth can see it. During a solar eclipse, you must be within a narrow path, where the Moon's shadow falls.
  • Lunar eclipses tend to last longer than solar eclipses. The maximum theoretical duration of totality in a total solar eclipse is 7:30 minutes, while totality in a total lunar eclipse can last up to 100 minutes.

Fun fact: On average, a total lunar eclipse can be seen from any given location every 2.5 years, while it takes about 375 years for a total solar eclipse to happen again at a specific location.

How Many Eclipses in a Year?

Most calendar years have 4 eclipses, which is the minimum number of eclipses that must take place in a year. Two of these 4 eclipses must be solar eclipses. While rare, the maximum number of eclipses that can take place in a calendar year is 7 (5 solar and 2 lunar eclipses or 2 solar and 5 lunar eclipses).

There can be at least 2 and at most 5 solar eclipses in a year. Out of these, no more than 2 eclipses can be total eclipses of the Sun. It is quite rare for a calendar year to have 5 solar eclipses.

According to NASA calculations, only about 25 years in the past 5000 years have had 5 solar eclipses. The last time this happened was in 1935, and the next time will be in 2206, when two solar eclipses will occur in December.

Similar Eclipses Every 18 Years

Solar eclipses tend to occur in cycles, called eclipse cycles. Astronomers and scientists use eclipse cycles to predict eclipses and calculate their features. One of the most popularly studied eclipse cycles is the Saros cycle. The ancient Babylonians used it to predict lunar eclipses.

The Saros cycle is a period of approximately 6,585.3 days or around 18 years, 11 days and 8 hours and occurs due to a combination of 3 lunar cycles:

  • The synodic month: the time it takes from one new Moon to another.
  • The anomalistic month: the time it takes from Moon perigee to perigee.
  • The draconic month: the time it takes from one lunar node to another.

Two solar eclipses separated by a Saros cycle have similar features – they occur at the same lunar node, with the Moon roughly at the same distance from the Earth. The eclipses also take place at about the same time of the year and around the same time of the day.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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