Eclipses, both solar and lunar, are highly memorable events for anyone with the slightest interest in astronomy or the universe.
Although most people have likely seen more total lunar eclipses than total solar eclipses, the two happen about equally often: NASA's eclipse catalogs for the 5,000 years from 2000 BCE to 3000 CE count 3,479 total lunar eclipses and 3,173 total solar eclipses, plus 569 hybrid eclipses that are total along part of their path.
The reason total solar eclipses are less often seen is due to the fact that they can only be witnessed along a narrow path on Earth that often crosses oceans or remote regions.
We experience at least two eclipse seasons every year separated by six 29.5 day lunar cycles. Since the eclipses of the Sun and the Moon typically come in pairs, in one calendar year as little as 4 eclipses can occur. On the other hand, as many as 7 eclipses can take place throughout a calendar year.
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How Do Eclipses Occur?
To gain an understanding of how eclipses occur, you must first understand what the ecliptic plane is.
The ecliptic plane, also known as the ecliptic, is a path that completely encircles the Earth. The Sun always sits on this path, and the Moon and the planets travel close to it. The Moon's orbit is tilted about 5° to the ecliptic, so the Moon crosses it only twice a month, at two points called nodes.
The 12 zodiac constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces) connect along this path as well. Under the modern constellation boundaries, the ecliptic also crosses a 13th constellation, Ophiuchus.
The positioning of the ecliptic, in relation to the equator, is tilted at a 23.5° angle.
An eclipse of the Sun or the Moon is based on specific Moon transits that intersect with the ecliptic. When the Moon is at its "New Moon" phase of its lunar cycle on or near the ecliptic, we experience a solar eclipse. When the Moon is at its "Full Moon" phase of its lunar cycle on or near the ecliptic, we experience a lunar eclipse. Either kind can be total or partial, depending on how closely the Sun, Earth and Moon line up, and a solar eclipse is annular instead of total when the Moon is near its farthest from Earth.
What is a Lunar Eclipse?
A lunar eclipse is an astronomical event that occurs when the Moon crosses the ecliptic plane at a lunar node during its full phase of its 29.5 day cycle. At that moment, from our perspective on Earth, the Moon appears opposite to the Sun in ecliptic longitude. The moment when the Sun, Earth and Moon are closely aligned is referred to as a syzygy.
During a total lunar eclipse, the Moon appears reddish in color due to the atmospheric refraction of the tiny amount of sunlight that reaches the Moon. The shorter wavelengths of blue light (450 to 495 nanometers) are scattered, while the longer wavelengths of red light (620 to 750 nanometers) reach the Moon. In the visible spectrum, blue light is more affected than red light. This is the same reason why, at sunset, the Sun takes on a deeper orange color than during the day time.
It is important to note that not all lunar eclipses are equal. The Earth's shadow is split into two parts, the umbra and the penumbra. The umbra represents the innermost and darkest part of a shadow, while the penumbra is the faint outer part of the shadow, where the Sun is only partly blocked. That distinction is helpful for understanding the three types of lunar eclipses:
- Penumbral lunar eclipse - occurs when the Moon passes only into the Earth's penumbra.
- Partial lunar eclipse - occurs when the Moon partially penetrates the Earth's umbra.
- Total lunar eclipse - occurs when the Moon entirely passes into the Earth's umbra. A total eclipse in which part of the Moon passes through the central axis of the umbra is called a central lunar eclipse.
The Danjon scale was devised for rating the overall darkness of lunar eclipses from L = 0 (very dark eclipse) to L = 1 (dark eclipse, gray or brownish) to L = 2 (deep red or rust-colored) to L = 3 (brick-red eclipse) to L = 4 (very bright copper-red eclipse).
When Is the Next Total Lunar Eclipse?
A total lunar eclipse is visible from anywhere on Earth where the Moon is above the horizon at the time, which is roughly half the planet. That makes it far easier to see than a total solar eclipse, whose total phase is visible only from a narrow path. Totality can last more than an hour.
For the date of the next one, check NASA's catalog of lunar eclipses from 2001 to 2100, where total eclipses are marked T. NASA's list of upcoming eclipses shows where the nearest eclipses of every type will be visible.
What is a Solar Eclipse?
A solar eclipse is an astronomical event that occurs when the Moon crosses the ecliptic plane at a lunar node during its new phase of its 29.5 day cycle. At that moment, from our perspective on Earth, the Moon and the Sun appear to be positioned at the same ecliptic longitude.
Experiencing a total solar eclipse is unmatched. During the eclipse you notice a drop in daylight to sudden darkness as the Sun's corona appears briefly at totality. The temperature decreases, many daytime animals fall quiet while night creatures such as crickets may start calling, and you are overwhelmed with the observation.
Although total solar eclipses occur every 18 months on average, they only recur at a given place on earth once every 360 to 410 years on average. The total solar eclipse may be the most exciting type to experience. However, there are four different types of solar eclipses you should know:
- Total solar eclipse - occurs when the Moon completely obscures the Sun.
- Annular solar eclipse - occurs when the Moon is in line with the Sun but appears smaller in size than the Sun, thereby displaying a bright ring.
- Hybrid solar eclipse - occurs when certain points on Earth observe the event as a total solar eclipse while other points on Earth observe the event as an annular solar eclipse.
- Partial solar eclipse - occurs when the Moon only partially obscures the Sun.
When Is the Next Solar Eclipse?
The path of totality is narrow and falls somewhere different each time, so seeing a total solar eclipse usually means traveling to it. For upcoming dates and where each eclipse will be visible, check NASA's list of upcoming eclipses.
Protect your eyes whenever you look at the Sun. The American Astronomical Society's eclipse eye-safety guidance says eclipse glasses must conform to the ISO 12312-2 standard, a telescope or binocular needs a solar filter attached securely to its front, and eclipse glasses do not make it safe to look through one without that filter. It says the only time to look at the Sun without a filter is totality, when the Moon completely covers the Sun's bright face, which happens only inside the narrow path of totality, and that this applies only to looking without optical aid: different rules apply to telescopes and binoculars.
Is it possible to observe the Sun during a Lunar Eclipse?
Yes, it is. This very rare sighting is referred to as a selenelion or horizontal eclipse. The Sun and the eclipsed Moon can be observed at the same time just before sunset or just after sunrise, when they both appear at opposite horizons.
Atmospheric refraction, the deviation of light from a straight line, causes each object to appear higher above the horizon in the sky than they actually are. Typically, only an observer with an unobstructed view of both horizons, such as from high ground or flat open terrain, may be able to witness this occurrence. If you watch the Moon through binoculars or a telescope, do not turn them toward the Sun: NASA warns that viewing any part of the bright Sun through a camera lens, binoculars, or a telescope without a special-purpose solar filter secured over the front of the optics will instantly cause severe eye injury.
What's next?
Now that you have a better understanding of lunar and solar eclipses, let's expand your astronomical knowledge towards the planets in our solar system. If you're interested in learning how to observe our celestial neighbors in the sky, check out How to Observe the Planets With a Telescope.