- An eclipse is the complete or partial obscuring of a celestial body by another other. An eclipse occurs when three celestial objects become aligned.
- The many eclipse phenomena known to astronomers are of two distinctly different types.
- In the first, the eclipsing body comes between an observer and the eclipsed object; the latter appears to the observer totally or partly covered by the eclipsing object.
- Eclipses of the Sun, occultations of stars by the Moon, transits of Venus or Mercury across the Sun’s disk, and eclipses of binary stars are of this kind.
- Eclipses of the second type affect only planets or natural satellites that are not self-luminous.
- In this case, the eclipsing body intervenes between the Sun and the eclipsed object.
- The latter remains in view of the observer, but its illumination by the Sun is interrupted, and it becomes darkened by entering into the shadow of the eclipsing object.
- Examples of this kind of eclipse phenomenon are eclipses of the Moon.
Sometimes when the Moon orbits the 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.
There are three main types of solar eclipses:
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.
- No sunlight penetrates the umbra, the inner part of the shadow.
- To observers on the Earth within the umbra, the disk of the Sun will appear completely covered by that of the Moon.
- Such a solar eclipse is said to be total. Because the umbra is narrow at its intersection with the Earth, a total eclipse can be observed only within a very narrow area–the zone of totality.
- Furthermore, because of the relative motion of the bodies, the conical shadow moves rapidly over the terrestrial surface; the totality of the solar eclipse thus lasts only a short time (less than eight minutes at any one place on the Earth).
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.
Annular (an-yə-lər) 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.
During a solar eclipse, the Moon casts two shadows on Earth.
- The umbra (əm-brə): This shadow gets smaller as it reaches Earth. It is the dark center of the Moon’s shadow. People standing in the umbra will see a total eclipse.
- The penumbra (pə-ˈnəm-brə): The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse.
Solar eclipses happen every 18 months somewhere on Earth. Unlike lunar eclipses, solar eclipses last only a few minutes.
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. (This sunlight is what causes the Moon to shine.) 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 can be seen from Earth at night. There are three types of lunar eclipses:
- Total lunar eclipse.
- Partial lunar eclipse
- Penumbral lunar eclipse
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.
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.
Penumbral lunar eclipse
- A penumbral lunar eclipse occurs when the Sun, Earth, and the Moon are imperfectly aligned.
- When this happens, the Earth blocks some of the Sun’s light from directly reaching the Moon’s surface and covers all or part of the Moon with the outer part of its shadow, also known as the penumbra.
- Since the penumbra is much fainter than the dark core of the Earth’s shadow, the umbra, a penumbral eclipse of the Moon is often difficult to tell apart from a normal Full Moon.
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.
- The study of Earth’s shadow projected on the Moon allows us to deduce that Earth is spherical. The ancient Greeks worked this out.
- The ancient Greeks and Romans used dated references to eclipses to improve the calendar.
- Using lunar eclipse timing, as far back as the third century BC, Aristarchus from Samos estimated the lunar diameter.Using Eratosthene’s previous measurement of Earth’s diameter, he deduced the Earth-Moon distance.
- Hipparcos (150 BC) and Ptolemeus (2nd century AD) improved with impressive precision the measurements of the lunar diameter and Earth-Moon distance.
- In the 17th century, in order to improve longitude determination, absolute cartography made use of lunar eclipse phenomena, which were observable simultaneously from different points.
- Today, during lunar eclipses, laser-ranging measurements can be made with great accuracy using reflectors placed on the Moon during the Apollo and Lunokhod missions. This has allowed more precise measurement of lunar acceleration and the slowing down in Earth’s rotation.
- Analysis of the refracted light of Earth’s atmosphere during lunar eclipses has also made it possible to show that atmospheric ozone is confined to a layer between 50 and 80 kilometres above Earth’s surface.
- Eclipses made it possible to determine with precision the shape of the Moon.
- Their study improved the prediction of ephemerides.
- Even today, a total solar eclipse still allows astrophysicists to make valuable scientific measurements, particularly when co-ordinated with measurements from observatories in space.
- Solar eclipses enable scientists to measure accurately the diameter of the Sun and to search for variations in that diameter over long time scales.
- Geophysicists measure eclipse phenomena induced in the high terrestrial atmosphere.
- Total solar eclipses allow the observation of structures of the solar corona that cannot usually be studied due to the higher normal luminosity of skylight during the day.
- The structures in the corona are similar to patterns seen around a magnet. In fact sunspots were shown to be solar surface magnetic structures, which have their counterpart in the corona.
- The study of the solar corona gives us much information about the Sun’s surface and its global variations.
- The morphology of the corona is changing due to the reorganisation of the surface magnetic field during the solar cycle, which can be seen in eclipse pictures taken at different epochs.
- The re-analysis of historical eclipse reports and documents could help to understand long term solar magnetic variations.
- Eclipses make it possible to diagnose the physical conditions of temperature (at more than 1 million degrees), densities and dynamics, both in the corona and at the base of the sources of the solar wind.