Total solar eclipses explained
During a total solar eclipse the Moon completely covers the Sun, as seen from Earth. The phenomenon can be observed when the Moon moves between the Earth and the Sun, and the three celestial bodies form a straight line: Earth - Moon - Sun.
Total solar eclipses are rare occurrences. They can only be observed during New Moon, when:
- the Sun is near one of the nodes of the lunar orbit, so Earth, Sun and Moon form a straight line,
- the Moon is at perigee,
- and the observer is located in the path of the Moon's umbra.
The Moon's orbit and lunar nodes
The Earth revolves around the Sun and the Moon circles the Earth. During New Moon, the Moon passes roughly between Earth and Sun. However, in most cases the three celestial bodies do not form a straight line, so the Sun is not eclipsed.
The reason why total solar eclipses do not happen every New Moon is that the lunar orbital plane - the imaginary flat surface whose outer rim is formed by the Moon's path around Earth - runs at an angle of approximately 5 degrees to the Earth's orbital plane around the Sun (ecliptic). The points where the two orbital planes meet are called lunar nodes. Only if the Sun appears near one of the two lunar nodes during New Moon can a total solar eclipse be observed from some locations on Earth.
Upcoming Total Solar Eclipses
|Dates||Visibility Map/Path of the eclipse|
|Mar 20, 2015|
|Mar 8 / Mar 9, 2016|
|Aug 21, 2017|
|Jul 2, 2019|
|Dec 14, 2020|
|Dec 4, 2021|
|Apr 20, 2023|
Lunar perigee and apogee
The Moon's orbit around Earth is not round but forms an ellipse. This is why the distance from Earth to the Moon varies constantly. The Moon is at perigee when it is closest to Earth. The point of greatest distance is called apogee.
Because of the varying distance, the Moon appears larger when it is at perigee. Although it is much smaller than the Sun, the Moon's apparent size then roughly matches the Sun's, as seen from Earth. For that reason, it can only cover all of the Sun's disk during a solar eclipse when it is at perigee.
The Moon's shadow
Like any other object's shadow, the Moon's shadow consists of three different areas: the innermost and darkest part (umbra), the lighter, outer part (penumbra), and an area beyond the umbra that is only visible from Earth when the Moon is at apogee (antumbra). To observe a total solar eclipse, the observer must be located in the umbra, which is only a few hundred kilometers wide and travels eastward at about 1,700 km/h (1,056 mph).
Phenomena observed during totality
Some phenomena can only be observed when the Moon completely obscures the Sun.
The Sun's corona
Only when the Sun's glaring center is completely obscured by the Moon can its hot plasma “atmosphere” - the corona - be seen as a faint ring of rays around the dark Moon.
About one minute before and after totality, moving wavy lines of alternating light and dark can be observed on plain-colored surfaces. These shadow bands are the result of the light emitted from a thin solar crescent being refracted by the Earth's atmophere.
Did you know...?
Totality of a solar eclipse can last up to 7 minutes and 31 seconds, but these eclipses are very rare. The next instance of a similarly long totality is predicted to occur in 2150, when the Sun is expected to be completely obscured for 7 minutes and 14 seconds - longer than during any total solar eclipse since the 9th century. In most cases, the period of totality is shorter than 5 minutes.
Even more rare is the simultaneous occurrence of a solar eclipse and the transit of a planet. The next time Venus is predicted to move in front of the Sun during a solar eclipse is on April 5 of the year 15232.
All about solar eclipses
- Types of solar and lunar eclipses
- Total solar eclipses
- Partial solar eclipses
- Annular solar eclipses
- Solar eclipses in history
- Eye safety during solar eclipses
- Make a pinhole projector
Watch daylight move across the planet... More