Shadows in space
The Sun lights Earth and the Moon from one side, so each casts a long shadow into space. Because the Sun is a wide disc rather than a point, every shadow has two parts. In the umbra, the dark core, the Sun is completely hidden. In the surrounding penumbra, only part of the Sun is hidden. An eclipse happens when one body moves into the other’s shadow.
Solar eclipses
A solar eclipse happens when the Moon passes between the Sun and Earth, at new moon. There are three main types:
- Total: the Moon completely covers the Sun. The sky darkens and the Sun’s faint outer atmosphere, the corona, appears. Totality is seen only inside the Moon’s umbra, a path on Earth usually less than about 270 km wide, and lasts at most about 7½ minutes.
- Annular: the Moon is near the far part of its orbit and looks slightly smaller than the Sun, so a bright “ring of fire” remains. The umbra ends before it reaches Earth.
- Partial: the Moon covers only part of the Sun, as seen from places in its penumbra, or when the alignment is not exact.
Total eclipses are possible because of a coincidence: the Sun is about 400 times wider than the Moon but also about 400 times farther away, so both appear about half a degree wide.
Lunar eclipses
A lunar eclipse happens when the Moon passes into Earth’s shadow, at full moon. In atotal lunar eclipse the whole Moon is in the umbra and often turns coppery red: sunlight passing through Earth’s atmosphere is bent into the shadow, and the blue part is scattered away, the same reason sunsets are red. In a partial eclipse only part of the Moon enters the umbra; in a penumbraleclipse it only grazes the penumbra and dims so slightly that many people do not notice. A lunar eclipse can be seen from everywhere on Earth’s night side and can last for hours.
Why not every month?
There is a new moon and a full moon every 29.5 days, but eclipses are much rarer. The reason is that the Moon’s orbit is tilted about 5° to the plane of Earth’s orbit around the Sun. The two places where the Moon’s orbit crosses that plane are called nodes. At most new moons the Moon passes slightly above or below the line from Earth to the Sun, and its shadow misses Earth; at most full moons it passes above or below Earth’s shadow. Five degrees sounds small, but at the Moon’s distance it is about ten Moon-widths.
Eclipse seasons
An eclipse needs a new or full moon that falls close to a node. That only happens when the line of nodes points roughly towards the Sun, which occurs twice a year, about 173 days apart. Each of these eclipse seasons lasts a little over a month, long enough to contain at least one new moon, so there is at least one solar eclipse each season, often with a lunar eclipse two weeks before or after. Every year has between four and seven eclipses in total. The line of nodes slowly turns, so the seasons drift earlier each year, and similar eclipses repeat after about 18 years and 11 days, a period called the saros.
Watching safely
Never look directly at the Sun, even when it is mostly covered, and never through binoculars, a telescope or a camera without a proper solar filter. Use certified eclipse glasses (meeting the ISO 12312-2 standard) or project the Sun’s image through a pinhole. Only during the brief total phase of a total solar eclipse is it safe to look without protection. Lunar eclipses are completely safe to watch.
Key terms
- Umbra
- The darkest part of a shadow, where the light source is completely blocked.
- Penumbra
- The outer part of a shadow, where the light source is only partly blocked.
- Node
- One of the two points where the Moon’s orbit crosses the plane of Earth’s orbit.
- Corona
- The Sun’s hot, faint outer atmosphere, visible during a total solar eclipse.
- Saros
- A cycle of about 18 years 11 days after which a similar eclipse repeats.
Eclipses in science and history
- Testing relativity: during the 1919 total eclipse, starlight bending around the Sun was measured, supporting Einstein’s theory of gravity.
- Discovering helium: the element was first identified in 1868 from the spectrum of the Sun’s edge during an eclipse.
- Earth is round: Aristotle noted that Earth’s shadow on the Moon during lunar eclipses is always curved.
- Dating history: eclipses recorded in ancient texts help historians pin down exact dates.
Common misconceptions
- “The phases of the Moon are caused by Earth’s shadow.” Phases come from our viewing angle; Earth’s shadow only matters during a lunar eclipse.
- “Eclipses happen every month.” The 5° tilt makes them happen only in eclipse seasons.
- “A partially covered Sun is safe to look at.” Even a thin crescent can damage your eyes.
- “A total solar eclipse is visible everywhere it is daytime.” Totality is only seen along a narrow path.
What the model simplifies
The eclipse type is calculated from real values: the sizes of the Sun, Earth and Moon, their distances, and the 5.145° tilt. For solar eclipses the view is from the best-placed spot on Earth, found by allowing for the Moon’s parallax; for lunar eclipses Earth’s shadow is enlarged by 2% for the atmosphere, as eclipse predictors do. The drawings are not to scale: Earth, the Moon and the heights in the side view are enlarged eight times so you can see them, while the Earth–Moon distance is to scale. The Moon’s orbit is drawn as a circle, and the “Varies” setting uses a simple sine-wave model of its changing distance. Real eclipse prediction also accounts for the Moon’s elliptical, perturbed orbit, Earth’s rotation and its shape, so this simulator does not predict real dates or paths.
Key takeaways
- Solar eclipses happen at new moon, when the Moon’s shadow falls on Earth; lunar eclipses at full moon, when the Moon enters Earth’s shadow.
- Shadows have a dark umbra and a paler penumbra; their sizes decide whether an eclipse is total, annular, partial or penumbral.
- The Moon’s 5° orbital tilt means eclipses only happen when a new or full moon falls near a node.
- Eclipses come in seasons about 173 days apart, with four to seven eclipses each year.
- Never look directly at the Sun without certified eye protection.