How often and when do they occur?
How often? There's not an easy answer. For an eclipse to occur the Sun, Moon and Earth have to line up in the same plane. This can only happen when the Moon is full or new and when the Moon passes through the ecliptic. Because the Moon's orbit is inclined 5 degrees to the Earth's, it spends most of its time below or above the plane of the Earth and the ecliptic.
A lunar eclipse line up could look like this ... Full Moon rising–Earth–Sun setting ... or like this ... Sun rising–Earth–Full Moon setting. In either case the Earth's shadow blocks out the "ecliptic" Full Moon. Note: Lunar eclipses, unlike solar eclipses, are completely safe to watch. Lunar Eclipses for Beginners
A solar eclipse line up could look like this Earth–New Moon setting-Sun setting ... or like this ... Sun rising-New Moon rising–Earth. In either case the "ecliptic" New Moon conjuncts and passes in front of the Sun blocking the Sun's light to the Earth. Solar Eclipses for Beginners Note: New Moons are invisible, hidden in the Sun's bright light, but during a solar eclipse the New Moon "appears" as a darkness covering some portion of the Sun. Viewing Solar Eclipses Safely
August 24, 1998 - An Annular Eclipse of the Sun
Image: Olivier Staiger
Total solar eclipses can be expected about every year and a half or so, but in any one calendar year, there are at least two and as many as five solar eclipses ... these can be partial, total and annular (see description under image) and often only visible from isolated regions of the Earth, which is why they seem so infrequent. In fact, solar eclipses outnumber lunar eclipses by almost 5 to 3, if you ignore the penumbral lunar eclipses that are essentially unobservable.
Total lunar eclipses come in clusters. There can be two or three during a period of a year or a year and a half, followed by a lull of two or three years before another round begins. When you add partial eclipses there can be three in a calendar year and again, it's quite possible to have none at all. Lunar eclipses are visible from the entire nighttime hemisphere of our planet and as a result are more frequently observed and seem to be more numerous.
Exceptions to this lunar eclipse rule ... Between May 15, 2003 and October 28, 2004 there were four total lunar eclipses — an eclipse occurring roughly at 6 month intervals (see Tetrads below) within less than a year and a half! Three of the four were visible from New York and other parts the U.S. East Coast. This flood of lunar eclipses made them seem commonplace. December 30, 1982 to August 18, 1989 was a seven year eclipse drought for the New York region, even though other parts of the world experienced several totalities! All of those eclipses occurred during the daytime when the Moon was below the New York region horizon.
Eclipses also tend to come in pairs with solar and lunar types complementing each other. Combining both solar and lunar eclipses, it's possible but rare for one calendar year to contain a maximum of seven eclipses. However, these can only occur in the combinations of five solar and two lunar, or four solar and three lunar. In 1982 there were four solar eclipses and three rare total lunar eclipses. This will not happen again until 2485 AD. The November 9, 2003 total lunar eclipse was visible across much of the United States, although like the May 15, 2003 total lunar eclipse, the Moon rose already in partial eclipse. After October 28, 2004, the next total lunar — visible anywhere on Earth — didn't occur until March 2007! Similar to the May 31, 2003 annular solar eclipse, North America missed out seeing the November 23, 2003 total solar eclipse. From articles by: Joe Rao and Fred Espenak
Schiaparelli Discovers Tetrads
Four consecutive total lunar eclipses, occurring at six-month intervals, are part of an uncommon lunar cycle known as a tetrad. Such a series ended October 2004 and another began April 2014.
Tetrad of Lunar Eclipses 2014-2015
2014-2015 Tetrad ~ "Blood Moons"
Mark Blitz and John Hagee, Christian pastors, apply the term Blood Moon to the 4 total eclipse Full Moons of the 2014-2015 tetrad. They feel this lunar tetrad represents a fulfillment of Biblical prophecy, when the moon is supposed to turn blood red before the end times, as described in Joel 2:31 (Common English Bible).
Though there are a total of 8 tetrads in the 21st century (2001 to 2100), the 2014-2015 tetrad is significant to the Biblical prophecy because it coincides with two important Jewish holidays: Passover and Tabernacles.
EarthSky.org says, "However, if we want to know which tetrads specifically fell on the Jewish feasts of Passover and Tabernacles, there appear to be a total of 8 in these 21 centuries ... [from 162-163 C.E. to 2014-2015 C.E.]."
Learn more from EarthSky.org at What is a Blood Moon?
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