EclipseGeeks.com - All you ever wanted to know about eclipses and other celstial bodies
Selenehelion or Selenelion Phenomena – sometimes called a Horizontal Eclipse
Lunar eclipses occurring just before sunrise or just after sunset produce an amazing phenomenon
A Selenehelion or selenelion event occurs during a Lunar Eclipse and can only be viewed just before sunset or just after sunrise when it is possible to simultaneously view the sun rising in the east and the eclipsed full moon setting in the west, both at the same time, appearing just above the horizon at opposite points in the sky. This has led to the event sometimes being referred to as a Horizontal Eclipse. It occurs during every lunar eclipse at all those places on the Earth where it is sunrise or sunset at the time of the eclipse. The reddened light that reaches the Moon comes from all the simultaneous sunrises and sunsets on the Earth and from the atmosphere of Earth bending light inwards towards the Moon.
The phenomena is not a type of eclipse, nor a sub classification of any type of eclipse, it is simple a phenomena which occurs during a lunar eclipse just before sunset or just after sunrise.
This phenomenon may seem like an impossibility - to be able to see both the Sun and Moon in the sky at the same time during a lunar eclipse. How is it possible? Depending on the observer’s location there is short window of one to a few minutes when it is possible to simultaneously view the sun rising in the east and the eclipsed full moon setting in the west. One or both bodies may actually be below the horizon but atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality.
A selenehelion phenomenon is best observed from high ground with clear unobstructed vision towards both ends of the horizon. Hills, mountains, or high ridges are all good vantage points to view this spectacular sight.
Other Planets and Moons can also have Eclipses - The moons of Mars - Phobos and Demois
Geometry of a Total Lunar Eclipse
An eclipse of the Moon, or lunar eclipse, is when the Earth is between the Sun and Moon and only occurs if the Moon passes through all or some portion of Earth's umbra shadow therefore blocking sunlight directly striking the Moon’s surface. This can occur only when the Sun, Earth, and Moon are aligned exactly, or almost exactly.
A lunar eclipse occurs at night and only when there is a Full Moon. A lunar eclipse can last for many hours, and can be seen from the entire night side of the Earth.
Geometry of a Total Solar Eclipse
An eclipse of the Sun, or a solar eclipse, is when the Moon is between the Sun and Earth and only occurs when the Moon is at just the right distance and angle in the sky to cover the Sun, this can only occur when the Sun, Moon and Earth are exactly aligned producing a Total Solar Eclipse. The Moon also has to be at or near one of its nodes; a node is simply the point at which the Moon crosses the eliptic from south to north or vice versa as it orbits the Earth.
A Total Solar Eclipse occurs during daytime and only when there is a New Moon. A solar eclipse duration is short with totality lasting from a few secounds to a few minutes. A Total Solar Eclipse is only seen by a minority of people along a narrow corridor and appears different according to ones location and distance from the central track of totality.
The Sun is 400 times the Moon's diameter, and 400 times as far away
It is an amazing coincidence that total solar eclipses actually occur. It is all due to size and distance between all three celestial bodies of the Sun, Moon and Earth; all being (as the Goldilocks Principle says) it's just right.
The Sun is 400 times the Moon's diameter and 400 times as far away. This means that the Sun and Moon appear to have the same apparent size in the sky when viewed from Earth; both having approximately the same degree of 0.5 arc in angular measurement.
A Total Solar Eclipse only occurs when the Moon is at or very near perigee (closest distance to Earth) and all three bodies of the Sun, Moon, and Earth are in alignment. When the Moon is at apogee (furthest distance from Earth) it is 11% more distant from Earth than it is a perigee, this makes the apparent size of the Moon slightly smaller in diameter. The difference is small but is enough to change the characteristics of a solar eclipse, resulting in that even if all three bodies of the Sun, Moon and Earth are in perfect alignment, the Moon’s diameter still remains too small to cover the disc of the Sun and an annular eclipse will occur.
The Moon also has to be at one of its nodes; a node is simply the point at which the Moon crosses the eliptic as it orbits the Earth. The orbit of the Moon around Earth is inclined at approximately about 5.1° to Earth's orbit around the Sun. As a consequence, the Moon's orbit crosses the ecliptic at two points / nodes two times every month, The ascending node is when the Moon crosses from the south to the north side of the ecliptic. The descending node is when the Moon crosses from the north to the south side of the ecliptic..
Mars two moons - Phobos and Deimos
Phobos is approximately 21km / 13 miles in diameter and orbits Mars in 7 hours, 39.2 minutes. A Martian day is 24 hours 37 minutes long and due to the rapid motion of Phobos, when conditions are correct, Phobos can create two eclipses per Martian day / Sols. The event is nevertheless still classified as an annular eclipse. An observer on the surface of Mars would never experience a solar eclipse for longer than about thirty seconds.
Deimos's diameter is approximately 12km / 7.5 miles and is too small and too far from Mars to cause an eclipse of any significance. The best an observer on Mars would see is a small object in transit across the surface of the Sun. Deimos would appear about as bright as the planet Venus looks from Earth and approximately the same size.
Mars does not have spectacular eclipses as occur on Earth and the motion of Deimos is regarded more of a transit rather than a true eclipse as observed from the surface of Mars. from the surface.
above; Phobos is the larger inner moon from Mars.
left; Deimos is the smaller outer moon from Mars.
Phobos translates as panic / fear. The word phobia is derived from the Greek word Phobos.
Deimos translates as; terror / fear.
images captured by the Mars Exploration Rover Opportunity. Image credit: NASA/JPL/Cornell
There are two primary types of eclipse which can be viewed from Earth:
a Lunar Eclipse and a Solar Eclipse
Definition of an Eclipse
An eclipse is the total or partial obscuring of one celestial body by another. It may occur when one celestial body passes in front of another therefore cutting off some or all of its light. It may also occur when a celestial body passes through all or part off the shadow of another celestial body.
Types of Eclipse - Solar Eclipses
There are four types of solar eclipse:
Total Solar Eclipse
Partial Solar Eclipse
Annular Solar Eclipse
Hybrid Solar Eclipse
Types of Eclipse - Lunar Eclipses
Total Lunar Eclipse
Partial Lunar Eclipse
Penumbral Lunar Eclipse
And a rare forth variation known as a;
Total Penumbral Lunar Eclipse
Click on diagrams of the Lunar Eclipse and Solar Eclipse to view an enlarged image.
Diameter of the Sun; 1,392,684km / 865,374 miles
Diameter of the Moon; 3,476km / 2,159 miles
Although the Sun is much bigger than the Moon they
both have the same apparant size in the sky both
having approximately the same degree of 0.5 arc in