Why the Sun sometimes disappears
A solar eclipse is simple geometry: the Moon passes directly between Earth and the Sun, casting its shadow onto Earth's surface. What varies, and what makes each eclipse different, is how completely that shadow blocks the Sun and how it does so.
Four kinds of solar eclipse
The type of eclipse you see from any one place depends on the alignment of the Sun, Moon and Earth, and on the Moon's distance from Earth at the time, since its orbit is not perfectly circular.
Total eclipse
The Moon fully covers the Sun's bright disc, revealing the corona, the Sun's faint outer atmosphere, which is normally invisible in daylight. Totality is only visible along a narrow path on Earth's surface, typically 100 to 250 kilometres wide, and usually lasts a few minutes at most.
Annular eclipse
The Moon is too far from Earth in its orbit to cover the Sun completely, leaving a bright ring, or "ring of fire", visible around its edge. Because the Sun is never fully blocked, safe solar filters are required throughout an annular eclipse, with no exception.
Partial eclipse
The Moon covers only part of the Sun's disc. Partial eclipses are visible over a much wider area than totality or annularity, and every total or annular eclipse is preceded and followed by a partial phase across the surrounding region.
Hybrid eclipse
A rare case in which the same eclipse appears total along one part of its path and annular along another, because the curve of Earth's surface brings some locations slightly closer to the Moon's shadow than others.
Centre line or the edge of the path?
Beyond weather, where you stand within the path of totality changes what you can see, a point made in detail by eclipse cartographer Xavier Jubier, a member of the International Astronomical Union's Working Group on Solar Eclipses.
Standing on the centre line gives the longest possible totality at that longitude, but the chromosphere, the reddish layer of the Sun's atmosphere just above its visible surface, is only exposed for a few seconds there. Move towards the edge of the path instead, and the Moon's disc grazes the Sun's edge far more slowly: the chromosphere can stay visible for 90 seconds or longer, the fleeting "shadow bands" that ripple across the ground beforehand are more likely to appear and last two to five times longer, and the diamond ring and Baily's beads effects, the last beads of sunlight breaking through lunar valleys, can last up to ten times longer, with several beads forming and fading in sequence rather than just one or two.
The trade-off is smaller than it sounds. Moving up to 20% of the way from the centre line towards the edge of the path typically shortens totality by only around 2%, since the exact duration at any point also depends on the Moon's uneven limb, its mountains and valleys, rather than distance from the centre line alone. Many experienced eclipse watchers therefore favour a position around 5% in from the edge of the path, trading a totality of roughly a third of the maximum for a longer, richer view of the Sun's edge effects.
Umbra, penumbra and antumbra
The Moon casts two, and sometimes three, distinct kinds of shadow, and which one falls on you determines what you see.
Umbra
The dark, central cone of the Moon's shadow. Anyone standing inside it experiences totality, with the Sun completely hidden.
Penumbra
The broader, lighter outer shadow surrounding the umbra. Observers here see a partial eclipse, the depth of which depends on how close they are to the umbra's edge.
Antumbra
Where the Moon is too distant for its umbra to reach Earth, its shadow narrows to a point before Earth's surface and then widens again. Observers within this extended shadow, the antumbra, see an annular eclipse rather than a total one.
The Saros cycle
Eclipses are not random. They recur in a pattern first recorded by Babylonian astronomers and now known as the Saros cycle, a period of approximately 18 years, 11 days and 8 hours after which the Sun, Moon and Earth return to a very similar relative geometry.
Each Saros cycle contains a family of related eclipses, spaced roughly one Saros apart, that gradually shift in path and character over centuries as the alignment slowly drifts. The total solar eclipse of 12 August 2026, for example, belongs to Saros series 126, while the total eclipse of 2 August 2027 belongs to Saros series 136. Astronomers use these series to predict eclipses many centuries in advance with high precision, since the underlying orbital mechanics are well understood and well tested against the historical record.
Viewing an eclipse safely
The Sun is dangerous to view directly at every stage of a partial or annular eclipse, and during the partial phases before and after totality in a total eclipse.
Looking directly at the Sun, even when most of it is covered by the Moon, can cause permanent retinal damage within seconds. The retina has no pain receptors, so damage can occur without any sensation of discomfort.
Certified solar filters
Eclipse glasses and handheld solar viewers should meet the ISO 12312-2 international safety standard, printed on genuine products. Inspect them for scratches or pinholes before use, and discard any that are damaged.
Pinhole projection
A simple and effective indirect method: pass sunlight through a small hole in card onto a second surface, projecting a safe image of the Sun's crescent without looking at it directly.
Filtered telescopes and binoculars
Any optical instrument used to view the Sun, including telescopes, binoculars and camera lenses, needs its own certified solar filter fitted over the front aperture. Eclipse glasses alone do not provide adequate protection when used with magnifying optics.
The exception: totality itself
During the brief minutes of total eclipse, when the Sun's bright disc is completely covered by the Moon, it is safe to view the corona directly with the naked eye. Filters should go back on the instant the first bright sliver of Sun reappears.