Solar and lunar eclipses: explanations, safety and traditions

Why an eclipse?

An eclipse occurs when the Earth, the Moon and the Sun are aligned. If the Moon passes between the Earth and the Sun, it hides all or part of the solar disc: this is a solar eclipse, which can only occur at new moon. If the Earth is between the Sun and the Moon, the Moon enters the shadow of our planet: this is a lunar eclipse, which can only occur at full moon.

Because the Moon's orbit is tilted about 5° relative to the Earth's, perfect alignment only happens when the new or full moon falls near one of the two points where the Moon's orbit crosses the plane of the Earth's orbit, called the nodes. This happens about twice a year, during "eclipse seasons" lasting about a month.

Solar eclipses

A coincidence makes solar eclipses spectacular: the Sun is about 400 times wider than the Moon, and also about 400 times farther away. Seen from the Earth, the two discs look almost the same size. Depending on the Moon's distance that day, you may see:

There are two to five solar eclipses a year worldwide. But totality only touches a narrow band, a few tens to a few hundred kilometres wide, and lasts at most 7 minutes 32 seconds. From any given place, a total eclipse is therefore rare: the last one in metropolitan France was on 11 August 1999 (the weather spoiled the show for many), and the next is calculated for 3 September 2081.

Watching a solar eclipse safely

Looking at the Sun unprotected can cause irreversible damage to the retina, without any pain at the time: the damage (solar retinopathy) may only appear a few hours later. Even when 99% of the disc is hidden, the remaining 1% is still dangerous. A few rules:

• Use only eclipse glasses that comply with ISO 12312-2, preferably bought from a known supplier, with the manufacturer's name and instructions. Discard them if the filter is scratched, pierced or peeling.
• Ordinary sunglasses, X-ray films, CDs or home-made smoked glass do not offer enough protection.
• Do not look through binoculars, a telescope or a camera without a specific solar filter placed in front of the instrument.
• The simplest method is projection. Make a small hole in a piece of card and project the image of the Sun onto a second card, with your back to the Sun. A colander has the same effect.
• Only the totality phase of a total eclipse can be watched without protection, and only when the Sun is entirely hidden.

Lunar eclipses

A lunar eclipse can be total (the Moon enters the Earth's shadow completely), partial, or penumbral (very subtle). During totality, which can last nearly 107 minutes, the Moon does not disappear: it takes on a coppery or red tint, sometimes called a "blood moon". This colour comes from the Earth's atmosphere, which bends red sunlight onto the Moon while scattering the blue: seen from the Moon, it is as if all the sunrises and sunsets on Earth were happening at once.

A lunar eclipse is visible wherever the Moon is above the horizon, and it is harmless to the naked eye. About one lunar eclipse in three is total. According to lists published by NASA, the next total lunar eclipse conveniently visible at night in France is that of 20 December 2029 (confirm it with an ephemeris site).

History, science and traditions

Eclipses have always captured the imagination. In ancient China, a solar eclipse was "the Sun being eaten" by a dragon, and people sought to predict them very early on. In Norse mythology, the wolves Sköll and Hati chase the Sun and the Moon. In Mesopotamia, a "substitute king" could be installed to divert the omen of a lunar eclipse. The Greek historian Herodotus reports that the philosopher Thales had predicted an eclipse that interrupted a battle between the Medes and the Lydians; it is often dated to 28 May 585 BCE, although this identification remains debated.

Science owes a lot to eclipses: helium was detected in the Sun's spectrum during the eclipse of 1868, and the observations of 1919 provided a famous test of Einstein's general relativity. The saros cycle, of about 18 years and 11 days (6,585.3 days), makes it possible to predict returns of similar eclipses.

In astrology, eclipses are read as moments of turning point or renewal. This is an interpretive tradition with no scientific basis; it can be enjoyed as a story, without attributing any power to it.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guide to the moon.