An eclipse is an astronomical phenomenon during which an astronomical object, like the Sun or the Moon, is partially or completely obscured for a short duration of time, by passing into the shadow of another body or by having another body passing between it and the viewer.
Solar and lunar eclipses occur when the Earth, the Sun, and the Moon are exactly or very closely aligned. During a solar eclipse, the Moon passes between the Sun and the Earth, blocking totally or partially the Sun’s rays and casting a shadow on parts of the Earth. Such events can only take place during a new moon. A lunar eclipse occurs when the Moon is on the far side of the Earth relative to the Sun and passes through the Earth’s shadow. This can only happen during a full moon. Lunar eclipses can be seen from the entire nightside half of the Earth surface, while solar eclipses, especially total solar eclipses, are only visible from a relatively small area on the Earth’s surface.
A shadow, for example during an eclipse, is divided into three parts, the umbra, penumbra and antumbra, created by a light source after hitting an opaque object. The umbra is the central region of the shadow and is the darkest part, where the light is completely blocked. In the penumbra light is only partially blocked. The antumbra is the part where the occluding body appears entirely within the disc of the light source.
Based on the occulted area of the sun, solar eclipses are classified into different types:
- During a partial solar eclipse, the Moon only obscures part of the Sun. All solar eclipses begin and end with a period of partial eclipse.
- Total solar eclipses take place when the Moon completely covers the Sun, allowing the solar corona to be visible. Total solar eclipses can only be seen along a narrow track on the Earth’s surface, called the path of totality.
- Like a total eclipse, annular solar eclipses occur when the Sun and the Moon are in line with the Earth. However, during an annular eclipse, the relative size of the moon is not large enough to completely block out the Sun, resulting in a very bright ring surrounding the Moon’s disk.
The size of a solar eclipse is measured by the fraction of solar disk diameter that is being occulted by the moon, which is not the same number as the solar disk area covered by the moon.
CAUTION: never look directly into the Sun. Even during solar eclipses, you need to use dedicated eclipse glasses.
Following the part of the Moon in the penumbra and or the umbra of the Earth, lunar eclipses can be classified into different categories:
- Penumbral lunar eclipses occur when Moon passes partly or completely into the Earth’s penumbra, but no part of the Moon is in the umbra. Each lunar eclipse begins and ends with a period of penumbral eclipse.
- Partial lunar eclipses occur when the Moon penetrates partially into the Earth’s umbra.
- Total lunar eclipses occur when the Moon passes entirely through the Earth’s umbra.
The Moon does not completely darken as it passes through the umbra because of the refraction of the sunlight by the Earth’s atmosphere into the shadow cone. Consequently, during a total lunar eclipse, the Moon takes on a reddish color instead of being completely dark.
The size of a lunar eclipse is measured by the fraction of the lunar disk diameter covered by the shadow or penumbral shadow, which is not the same number as the lunar disk area covered by the Earth’s shadow.
Because the orbit of the Earth around the Sun and the orbit of the Moon around the Earth don’t occur in the same plane, eclipses only take place during eclipse seasons roughly every six months. During the eclipse season, the lunar node – the line where the Moon’s orbital plane intersects with the Earth orbital plane – aligns or closely aligns with the Sun’s and Earth, so a solar eclipse is formed during the new moon phase and a lunar eclipse is formed during the full moon phase. Two of three eclipses happen every eclipse season, which lasts roughly 31 to 37 days.