Lunar Eclipse Tetrad Expected To Begin In April
April 14, 2014

Lunar Eclipse Expected On April 15

Brett Smith for - Your Universe Online

Skywatchers across North America will want to mark their calendars for April 15 – the next lunar eclipse.

Expected to be visible across the entire United States, weather permitting, the upcoming eclipse is the first of four total eclipses expected to occur over the next year-and-a-half at two-month intervals – a phenomenon known as a tetrad.

There are three types of lunar eclipses: penumbral, partial and total. A penumbral eclipse happens when the Moon passes across the pale borders of Earth’s shadow. The effect is so subtle that most skywatchers have trouble noticing the eclipse is even taking place. A partial eclipse occurs when the Moon partially passes into Earth’s shadow, darkening only a fraction of the Moon. A total eclipse is the most dramatic and is known for slowly coloring the Moon a dark shade of red.

[ Watch the Video: A Tetrad Of Lunar Eclipses About To Begin ]

These types of eclipses occur in a seemingly random order – making a tetrad a fairly rare occurrence over the course of thousands of years. The April 15th eclipse is followed by another eclipse on Oct. 8, 2014, and another on April 4, 2015, and another on Sept. 28 2015.

"During the 21st century, there are 9 sets of tetrads, so I would describe tetrads as a frequent occurrence in the current pattern of lunar eclipses," said NASA eclipse expert Fred Espenak. "But this has not always been the case. During the three hundred year interval from 1600 to 1900, for instance, there were no tetrads at all."

"The most unique thing about the 2014-2015 tetrad is that all of (eclipses) are visible for all or parts of the USA," he added.

The upcoming eclipse starts at 2 a.m. EST and totality, when the Moon will appear completely red, is expected to take place around 3 a.m. Totality is expected to end around 4:25 a.m.

The Moon appears red because sunlight passing through the Earth’s atmosphere becomes tinted red, as seen at each sunrise and sunset. During a total eclipse, this red light from the Earth’s atmosphere reaches the Moon – coloring it red.

Scientists have found that measuring the distance between the Earth and Moon using a laser is actually easier during a lunar eclipse. According to a report published in February in the journal Icarus, heat from the Sun causes a thermal gradient on the moon – disrupting the emissions of a laser rangefinder.

Researchers at Apache Point Observatory had been sending laser pulses of 100 quadrillion photons toward special reflectors placed on the Moon by Apollo astronauts. However, only about one photon would even return from each pulse sent during a full moon. The researchers suspected that their diminished returns on full moon nights were being caused by heating from the Sun, meaning that a lunar eclipse should boost the signal as soon as the surface cools.

During a lunar eclipse in 2010, the scientists ranged lasers from the three Apollo reflector arrays and an array mounted on a Soviet rover for five and a half hours as the edge of the Earth’s shadow passed by each of them in turn and as they re-emerged into the sunlight.

The team observed a tenfold spike in performance as the Sun’s light was blocked, just as they had predicted. The signal return was restored to the levels seen on other nights.