Solar Eclipses
When the Moon passes between Earth and the Sun.
Overview
A solar eclipse happens when the Moon passes between the Sun and Earth and casts its shadow on the surface. It is possible only because of a coincidence: the Moon and the Sun appear almost exactly the same size in our sky. Never look at the Sun without proper protection.
- The Sun is about 400 times wider than the Moon and about 400 times further away — an accident of the present epoch.
- Eclipses do not happen monthly because the Moon's orbit is tilted about five degrees.
- Totality is a genuinely different phenomenon from a 99% partial eclipse, not a slightly better version of it.
- Only during totality is it safe to look without a filter — at every other moment it is not.
The coincidence that makes totality possible
The Sun's diameter is roughly 400 times the Moon's, and it is roughly 400 times further away. The two therefore subtend almost the same angle in our sky — about half a degree — which is why the Moon can cover the Sun's disc almost exactly, revealing the corona without blocking it.
There is nothing necessary about this. The Moon is receding from Earth by a few centimetres a year, and in the distant future it will be too small in the sky to produce a total eclipse. We happen to live during the era when it works.
The four types
- Total: the Moon completely covers the photosphere, revealing the corona. Visible only from a narrow path, typically a few hundred kilometres wide, and lasting a few minutes at most at any one location.
- Annular: the Moon is near the far point of its elliptical orbit and appears slightly too small, leaving a bright ring of photosphere visible. The corona is not visible, and the sky does not darken appreciably.
- Partial: the observer is outside the central path and sees the Moon cover only part of the Sun.
- Hybrid: a rare case where the eclipse is annular at some points along the path and total at others, because of Earth's curvature.
Why not every month
The Moon passes between Earth and the Sun once per synodic month, so a naive expectation is a solar eclipse every new Moon. It does not happen because the Moon's orbital plane is tilted by about five degrees relative to Earth's orbital plane, so the Moon usually passes above or below the Sun as seen from Earth.
An eclipse requires new Moon to occur near a node — one of the two points where the orbital planes intersect. That alignment recurs during eclipse seasons roughly every six months, and the geometry repeats approximately every 18 years and 11 days: the Saros cycle, which is how Babylonian astronomers predicted eclipses without knowing what caused them.
Safety, stated plainly
Looking at the Sun without proper protection can cause permanent retinal damage, and because the retina has no pain receptors, the injury occurs without any sensation of harm. Sunglasses, exposed film, smoked glass and improvised filters are not adequate. Use eclipse glasses meeting the ISO 12312-2 standard, or a properly designed solar filter fitted over the front of any optical instrument.
Never point binoculars or a telescope at the Sun without a purpose-made front-mounted filter — the concentrated light can cause severe injury in a fraction of a second and will destroy eyepiece-mounted filters. Pinhole projection onto a card is completely safe and requires no equipment. The sole exception is the brief window of totality, when the photosphere is entirely covered; the moment the first sliver reappears, protection is required again.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why isn't there a solar eclipse every month?
- Because the Moon's orbit is tilted about five degrees to Earth's orbital plane, so at most new Moons it passes above or below the Sun from our viewpoint. An eclipse needs new Moon to coincide with the Moon crossing one of the two nodes where the planes intersect, which happens during eclipse seasons roughly every six months.
- Is a 99% partial eclipse nearly as good as totality?
- No — they are qualitatively different. Even one percent of the Sun's disc remains overwhelmingly bright, so the corona stays invisible, the sky does not truly darken, and filters remain mandatory throughout. Totality reveals the corona, drops the temperature noticeably, and brings out bright stars and planets. The difference is a change of kind, not degree.
- How can I watch a solar eclipse safely?
- With eclipse glasses meeting the ISO 12312-2 standard, or a purpose-made solar filter mounted on the front of a telescope or binoculars, or by pinhole projection onto a card. Sunglasses, exposed film and smoked glass are not safe. Never use an unfiltered optical instrument. The only unprotected viewing is during totality itself, and protection must go back on the instant the Sun's edge reappears.
- What is an annular eclipse?
- One where the Moon is near apogee — the far point of its elliptical orbit — and so appears slightly smaller than the Sun. Instead of covering the disc completely, it leaves a bright ring of photosphere visible around it. The corona cannot be seen, the sky does not darken much, and eye protection is required throughout.