By Chris Deziel
Updated Mar 24, 2022
Dmytro Balkhovitin/iStock/GettyImages
The apparent diameters of the Sun and Moon match almost perfectly when viewed from Earth. This coincidence allows the Moon to fully obscure the Sun during a solar eclipse, revealing the Sun’s corona. The alignment is so precise that even small variations in distance can change the eclipse type.
Solar eclipses happen at new Moon, while lunar eclipses occur at full Moon when Earth lies between the Sun and Moon. The Moon’s orbital plane is tilted 5.1° relative to Earth’s orbit, so eclipses only occur when the Sun is within about 17° of one of the Moon’s nodes. This alignment defines the eclipse season, lasting roughly 34 days as the Sun moves 0.99° per day.
During an eclipse season, at least one solar and one lunar eclipse occur, but because the season spans more than a month, two solar or two lunar eclipses can happen within a single season.
The eclipse observed depends on three factors:
These variables produce:
The Earth’s orbit varies by about 5 million km between aphelion and perihelion, altering the Sun’s apparent size by ~1 arc‑minute. The Moon’s distance changes by ~50,000 km between apogee and perigee, shifting its apparent diameter by ~4 arc‑minutes (≈13% of its average size). Because the Moon’s size changes more, it has a greater effect on eclipse type.
An annular eclipse occurs when the Moon appears smaller than the Sun—typically when Earth is near perihelion (January) and the Moon is at apogee, or when the Moon is at apogee in July. If the Moon is at perigee (a “supermoon”), an annular eclipse cannot occur regardless of the time of year.
During annularity, the Sun’s corona remains visible as a “ring of fire,” and the sky takes on a faint twilight. Direct viewing of the Sun during an annular eclipse is more hazardous than during a total eclipse, so proper eye protection is essential.
In a total eclipse, the Moon’s shadow (umbra) forms a cone that intersects the Earth, creating a ~100‑mile‑wide path of totality that moves across the surface at 1,000–3,000 mph depending on latitude. Viewers inside this path experience up to 7½ minutes of totality before the umbra shifts eastward.
In an annular eclipse, the umbra never reaches the surface; instead, it comes to a focus above the Earth. The Sun’s outer ring remains visible beyond this focal point, illuminating the surrounding area. Annular eclipses can last up to 12½ minutes because the Moon’s smaller apparent size allows it to traverse the Sun’s disc more slowly.
During a lunar eclipse, the Earth’s umbra (1.4 million km long) completely blocks the Sun. As the Earth passes between the Sun and Moon, sunlight refracts through Earth’s atmosphere, casting a red hue on the Moon—hence the term “blood moon.” The totality phase can last up to 1 hour 40 minutes, with the overall event spanning up to 6 hours.
Ancient astronomers identified the Saros cycle—an 18‑year, 11‑day, 8‑hour interval—after which the Sun, Moon, and Earth return to nearly the same relative geometry, producing similar eclipses. NASA’s eclipse calendars project events well into the year 3000, allowing observers worldwide to anticipate future spectacular eclipses.
