As seen from the Earth, a solar eclipse is a type of eclipse that occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun. This can happen only at new moon when the Sun and the Moon are in conjunction as seen from Earth in an alignment referred to as syzygy. In a total eclipse, the disk of the Sun is fully obscured by the Moon.
If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every month. However, the Moon’s orbit is inclined (tilted) at more than 5 degrees to the Earth’s orbit around the Sun (see ecliptic), so its shadow at new moon usually misses Earth. Earth’s orbit is called the ecliptic plane as the Moon’s orbit must cross this plane in order for an eclipse (both solar as well as lunar) to occur. In addition, the Moon’s actual orbit is elliptical, often taking it far enough away from Earth that its apparent size is not large enough to block the Sun totally. The orbital planes cross each other at a line of nodes resulting in at least two, and up to five, solar eclipses occurring each year; no more than two of which can be total eclipses. However, total solar eclipses are rare at any particular location because totality exists only along a narrow path on the Earth’s surface traced by the Moon’s shadow or umbra.
Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse.
Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, it is estimated that they recur at any given place only once every 360 to 410 years, on average. The total eclipse lasts for only a maximum of a few minutes at any location, because the Moon’s umbra moves eastward at over 1700 km/h.
Frequency per year
Between two and five solar eclipses occur every year, with at least one per eclipse season. On average, there are about 240 solar eclipses each century.
Total solar eclipses are seen on Earth because of a fortuitous combination of circumstances. Even on Earth, the diversity of eclipses familiar to people today is a temporary (on a geological time scale) phenomenon. Hundreds of millions of years in the past, the Moon was closer to the Earth and therefore apparently larger, so every solar eclipse was total and there were no annular eclipses. Over a billion years in the future, the Moon will be too far away to fully occlude the Sun, and no total eclipses will occur.
Due to tidal acceleration, the orbit of the Moon around the Earth becomes about 2.2 cm more distant each year. It is estimated that, in slightly less than 1.4 billion years, the distance from the Earth to the Moon will have increased by 30,400 km. During that period, the apparent angular diameter of the Moon will decrease in size, meaning that it will no longer be able to completely cover the Sun’s disk as seen from the Earth. This will be true even when the Moon is at perigee, and the Earth at aphelion. Moreover, the Sun is increasing in diameter by about 5% per billion years. Therefore, the last total solar eclipse on Earth will occur about six hundred million years from now.
Looking directly at the photosphere of the Sun (the bright disk of the Sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring.
Under normal conditions, the Sun is so bright that it is difficult to stare at it directly. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare at it. Looking at the Sun during an eclipse is as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun’s disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse). Viewing the Sun’s disk through any kind of optical aid (binoculars, a telescope, or even an optical camera viewfinder) is extremely hazardous and can cause irreversible eye damage within a fraction of a second.
Sunglasses do not make viewing the Sun safe. Only properly designed and certified solar filters should be used for direct viewing of the Sun’s disk. Especially, self-made filters using common objects such as a floppy disk removed from its case, a Compact Disc, a black color slide film, smoked glass, etc. must be avoided.
- A total solar eclipse occurs somewhere on Earth once in every 18 months on average.
- In any one location, a total solar eclipse is very rare, occurring on average once every 375 years.
- During a total solar eclipse, the Moon’s shadow is cast upon the Earth. There are two parts to this shadow – an outer shadow that covers a wide region creating a partial eclipse, and a much smaller central shadow that creates the total eclipse. As the Earth rotates, the central shadow creates a thin path known as the path of totality.
- Those outside of the path of totality will experience a partial eclipse, an event nowhere near as dramatic as a total eclipse.
- Even if only 1% of the Sun is visible, it is still 10,000 times too bright to see the exciting eclipse phenomena. You must be within the path of totality to feel the full experience.
- Even those who know what is happening can be caught off guard by a total solar eclipse. It is eerie, awe-‐inspiring, unsettling, beautiful, and often emotionally overwhelming.
- It is essential to consider eye safety when planning for the eclipse. People must use solar filters to view the partial phases of the eclipse.
- Totality can be viewed safely with the naked eye, but only if you are in the path of totality.