Solar eclipse: the Sun, the Moon and the shadow

Solar eclipses: perfect alignments

A historic total eclipse in Europe

On August 11, 1999, a total solar eclipse captivated Europe, Turkey and India. This rare phenomenon occurs when the Moon comes exactly between the Earth and the Sun. The shadow cast by the Moon on the Earth's surface temporarily plunged certain regions into daytime darkness.

The path of the lunar shadow

The eclipse began off the Canadian coast and crossed Western Europe as far as India. In France, it was total for about 2 minutes, with maximum occultation near Reims at 12:23 UTC. This event was the last of this intensity in the 20th century visible from continental Europe.

N.B.:
The next total eclipse to pass over Europe will occur on September 3, 2081. The path of totality will run a little farther south and be wider than this one. The shadow will cross France, Switzerland, Germany, Italy, Austria, Slovenia, Romania, Bulgaria, Turkey, Syria, Iraq, Iran and then the Indian Ocean, ending between the islands of Sumatra and Java.

The last 10 total solar eclipses

The table below lists the ten most recent total solar eclipses observed on Earth. The maximum duration corresponds to the longest time during which the solar disk was entirely occulted by the Moon at a given point along the path of the shadow.

Black Sun: the phenomenon of total eclipses

Conditions for Sun-Earth-Moon alignment

A solar eclipse occurs only during a new moon, when the Sun, the Moon and the Earth are practically aligned on the plane of the ecliptic. Because of the Moon's orbital inclination (5.1° relative to the plane of Earth's orbit), these alignments are exceptional. The apparent diameter of the Moon can be enough to totally occult the solar disk: this is a total eclipse. The lunar shadow zone, narrow, crosses the Earth's surface at several thousand kilometers per hour.

Umbra and penumbra: the zones of the eclipse

The shadow cast by the Moon breaks down into several distinct regions. The central zone, called the umbral shadow, is a converging cone where the solar disk is entirely masked: this is where the eclipse is total. Around it extends a vast region of penumbra, where only part of the Sun is occulted, producing a partial eclipse. Between these two zones, the shadow cone may be prolonged by an antumbra, responsible for annular eclipses when the Moon is too far away to cover the entire solar disk. The width of the umbral shadow on the ground rarely exceeds 200 km, while the penumbra covers several thousand kilometers. This shadow moves at high speed, often more than 2,000 km/h, and its shape varies according to the inclination of the solar rays and the terrestrial relief. Predictive calculations of an eclipse involve celestial mechanics, the orbital parameters of the Moon and the Earth, as well as the dynamics of elliptical orbits. They make it possible to determine with precision the trajectory, duration and local times of the different phases.

When day becomes night: the spectacle of solar eclipses

Observing the solar corona

The solar eclipse of August 11, 1999 offered a precious opportunity to observe the solar corona, invisible outside these events. The rapid cooling of the ambient air, the silence of the birds, and the drop in luminosity recall the complexity of the interactions between celestial objects and their effects on our environment.

Effects on the environment and scientific measurements

From a scientific point of view, measurements of temperature, radiation, and ionospheric activity were carried out in real time. This phenomenon was also massively observed by the public and covered by the media, underlining its cultural as much as scientific impact.

Why does the Moon perfectly mask the Sun?

A cosmic coincidence: the 400 ratio

It is a remarkable coincidence of celestial mechanics: the Sun is about 400 times larger than the Moon, but it is also about 400 times farther from the Earth. Thus, their apparent diameters seen from Earth are almost equal (about 0.5 degree or 30 arc minutes).

Calculating the apparent diameter

The formula for apparent diameter is: \( \theta \approx \frac{D}{d} \) where \( D \) is the real diameter and \( d \) the distance to the observer.
- For the Sun: \( D = 1.39 \times 10^6 \) km, \( d = 1.496 \times 10^8 \) km → \( \theta_{\text{Sun}} \approx 0.0093 \) rad.
- For the Moon: \( D = 3.474 \times 10^3 \) km, \( d = 3.844 \times 10^5 \) km → \( \theta_{\text{Moon}} \approx 0.0090 \) rad.
These values explain why the Moon can perfectly mask the Sun, thus allowing total or annular eclipses depending on its distance from the Earth (perigee or apogee).

A temporary phenomenon in Earth's history

The gradual recession of the Moon

The Moon is slowly moving away from the Earth, at a rate of about 3.8 cm per year, due to tidal effects. This orbital drift means that in a few hundred million years, the Moon will appear too small to completely mask the Sun.

The future disappearance of total eclipses

Total eclipses will disappear in favor of annular eclipses. We are therefore living in an exceptional period in geological history, when orbital conditions allow total eclipses, events that are both rare, spectacular and charged with scientific meaning.

Baily's beads: the pearls of light of a total solar eclipse

What are Baily's beads?

Baily's beads are visible, not on a lunar eclipse but during a solar eclipse by the Moon. During an annular solar eclipse one can observe this phenomenon called Baily's pearls. This phenomenon is due to the topography of the Moon's surface (presence of mountains, craters, valleys, etc.). It was described strikingly by Francis Baily (1774-1844), who noted this phenomenon in 1836.

The red flash and the diamond ring

These scratches of the Moon by the Sun's light during a solar eclipse allow the pearls of sunlight to shine in some places and not in others. The irregularities of the lunar profile are perfectly known and astronomers can calculate in advance the appearance of the beads on the eclipse. Baily's beads appear briefly for a few seconds along the eclipse path. Their duration is, near the edges, 1 to 2 minutes. The Red Flash appears just after the sight of Baily's beads. This very brief passage colors the solar corona bright pink.

The red color is due to the hydrogen alpha line, which radiates at this wavelength. The diamond ring or Airy's diamond is the last portion of the Sun emerging behind the black disk of the Moon. At that moment, this image resembles a ring adorned with a diamond shining with a thousand fires. It occurs just before or just after the total eclipse.