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, total solar eclipse occurrences are actually more common.
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 know as the ecliptic, is a path that completely encircles the Earth. Along this path you will find the Sun, the Moon, and the planets as they travel across the night sky.
The 12 zodiac constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces) connect along this path as well.
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 the ecliptic, we experience a total solar eclipse. When the Moon is at its "Full Moon" phase of its lunar cycle on the ecliptic, we experience a total lunar eclipse.
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 represents only a portion of a shadow. That distinction is helpful for understanding the four 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.
- Central lunar eclipse - occurs when the Moon passes through the center of the Earth's shadow.
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 and where it will be visible, check NASA's list of upcoming eclipses.
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, the animals become quiet 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. 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. Eclipse glasses should meet the ISO 12312-2 standard. A telescope or binocular needs a solar filter over the front of the instrument, and eclipse glasses do not make it safe to look through one without that filter. The only safe time to look without protection is totality itself, while the Moon completely covers the Sun.
Is it possible to observe the Sun during a Lunar Eclipse?
Yes, it is. This very rare sighting is referred to as a selenhelion 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.
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.