Photo: Michael S Adler · CC BY-SA 4.0
400x
Total solar eclipses happen because the Sun is about 400 times wider than the Moon and about 400 times farther away.
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The short answer
Yes. The Sun's diameter is roughly 400 times the Moon's, and it is roughly 400 times farther from Earth. The two effects nearly cancel, so both appear about half a degree wide, letting the Moon cover the Sun exactly.
Why it's true
How big something looks depends on its real size and its distance. The Sun is about 1.39 million kilometers across, and the Moon about 3,474 kilometers, a ratio of about 400. On average, the Sun is about 150 million kilometers away, and the Moon about 384,000, again a ratio of nearly 400. So in our sky, they look almost exactly the same size.
When the Moon passes directly between Earth and the Sun, it can block the bright solar disc completely, revealing the Sun's faint outer atmosphere, the corona. This total eclipse is only visible along a narrow path on Earth's surface. Totality never lasts more than about seven and a half minutes at one place.
No other planet in the solar system is known to have a moon that matches the Sun's size in its sky so closely.
By the numbers
~400
Sun to Moon size and distance ratio
~0.5°
Apparent width of each in the sky
~7.5 min
Maximum length of totality
~18 months
Typical gap between total eclipses somewhere on Earth
How we know
The sizes and distances of the Sun and Moon were measured with increasing precision from ancient Greek attempts to modern radar and lasers. The Moon's distance is now known to within centimeters thanks to reflectors left by Apollo astronauts, which bounce back laser beams fired from Earth.
Because both orbits are slightly oval, the match is not perfect. When the Moon is near its farthest point, it looks slightly smaller than the Sun, and an eclipse leaves a bright ring of sunlight around it. This is called an annular eclipse, or ring of fire. When the Moon is nearer, it covers the Sun completely, producing a total eclipse.
Chasing the shadow
The Moon's shadow races across Earth at more than 1,500 kilometers per hour. Eclipse chasers travel the world to stand in its path.
Scientists have even flown supersonic jets along the path, including Concorde in 1973, to stretch totality to more than an hour.
Worth remembering
- The Sun is about 400 times wider and 400 times farther than the Moon.
- That makes them look nearly the same size, enabling total eclipses.
- As the Moon drifts away, total eclipses will eventually end.
In context
Eclipses have shaped science as well as stirring awe. In 1868, observers studying an eclipse spotted a new spectral line in the Sun's light, which led to the discovery of helium. In 1919, eclipse measurements of starlight bending near the Sun gave early support to Einstein's general theory of relativity.
Ancient cultures also tracked eclipses carefully. Babylonian astronomers recognised a cycle, now called the saros, of about 18 years, after which similar eclipses repeat.
A closer look
What you see during totality
As the last sunlight vanishes, the sky darkens to a deep twilight. Bright planets and stars may appear, and the temperature can drop noticeably.
The pearly corona spreads out around the black Moon, and pink prominences may glow at the edge. Animals sometimes behave as if night has come.
Why not every month?
The Moon passes between Earth and the Sun every month at new moon, but its orbit is tilted about five degrees to Earth's.
Usually it passes slightly above or below the Sun. Eclipses happen only when the alignment is just right.
A temporary coincidence
Billions of years ago, the Moon was closer and looked bigger, so total eclipses were more common and annular ones rarer.
In the far future, as it drifts away, only annular eclipses will remain.
Related facts
Why total eclipses are temporary the Moon is drifting away.
An eclipse helped make that discovery helium was found in sunlight first.
More about the Moon's motion we always see the same side of the Moon.
For a sense of the Sun's true scale the Sun holds 99.8 percent of the solar system's mass.
Key terms
- Corona
- The Sun's faint outer atmosphere, visible during a total eclipse.
- Totality
- The period when the Moon completely covers the Sun's bright disc.
- Saros cycle
- A period of about 18 years after which similar eclipses repeat.
Myth vs reality
Myth
Total solar eclipses are extremely rare events worldwide.
Reality
One happens somewhere on Earth about every 18 months. Any single place sees one only rarely.
People also ask
Why can the Moon cover the Sun?
The Sun is about 400 times wider than the Moon but also about 400 times farther away, so both look roughly the same size from Earth.
What is an annular eclipse?
An eclipse that happens when the Moon is farther from Earth and looks slightly smaller than the Sun, leaving a bright ring of sunlight visible around it.
How long does a total solar eclipse last?
Totality usually lasts a few minutes. The theoretical maximum at any one location is about seven and a half minutes.
Will total eclipses always happen?
No. The Moon moves about 3.8 centimeters farther away each year. In hundreds of millions of years it will look too small to cover the Sun completely.
Is it safe to look at a solar eclipse?
Only with proper eclipse glasses or filters, except during the brief moment of totality. Looking at the partially covered Sun can damage your eyes.
