SOLAR and LUNAR ECLIPSES : Part 2

LUNAR ECLIPSES

Lunar eclipses occur because the monthly orbital path of the Moon can sometimes cause the illuminated lunar disk to disappear behind the projected shadow of the Earth for a short time. They only can happen during the just few minutes or several hours around the time of the Full Moon.

Unlike solar eclipses, lunar eclipses are slightly more common due to the greater apparent size of the Earth’s shadow as projected directly diametrically away from the Sun. As such, lunar eclipses roughly occur on average once or twice each yearly, with at least one occurring in the same part of the world once every one, two or three years.

Duration of any observed lunar eclipse can last as long as two hours fifty minutes, with totality lasting about half of this, or about one hour forty minutes.

Three Types of Lunar Eclipses

There are three types of solar eclipses.

Total, Partial and Penumbral

Components of the observed shadows appear as two distinct parts. These are the umbra and penumbra, and thirdly, the graduated shadow boundary that lies between the umbra and penumbra — the eclipse terminator.

 Figure 3.1: Type and Nature of Lunar Eclipses

Shows the lunar path as it enters and exits the Earth’s shadow during total, partial or penumbral lunar eclipses.

1) UMBRA Darkest portion seen during lunar eclipses. When the Moon passes through this shadow, it does not completely disappear from view, as sunlight from the Earth’s atmosphere refracts through it then is projected onto the surface of the Moon. This gives the Moon some illumination, making the umbral colour is usually as dull brick red to deep red to deep orange, with the depth of colour mostly depending on the slight variable distance between Earth and Moon. This is also related to the lunar distance when immersed within the shadow. As such, the orbital inclination of the Moon does not always travel through the mid-point of the Earth’ shadow, so we see only a partial lunar eclipse.

2) PENUMBRA : Produced by the secondary shadow of the Earth as projected onto the Moon. The best way to imagine how it is created, is to see it from the lunar perspective. From the Moon, the Sun would appear partially obscured or eclipsed by the round bulk of the Earth, causing the amount of sunlight to be reduced in intensity. We therefore see to slight drop in intensity compared to the brightness of Full Moon. It is usually impossible to see this visually, unless you know about the event beforehand. Penumbral eclipses can only be observed with suitable electronic light-sensitive equipment, and can be measured during the partial lunar eclipse phase, or even just before or after each total lunar eclipse.

3) ECLIPSE TERMINATOR : Dividing between the umbra and penumbra is the thin transitional region that sometimes appearing as an odd-looking group of colour bands. Some of these shadow bands maybe seen as only grey, but sometimes they appear like the familiar colours of the rainbow, except in the reverse order.

Each lunar eclipse path is seen from by about roughly half of the sphere of the Earth. In specific locations, often daylight will intervene, and this may reduce this percentage of the Earth that will see the phenomena considerably.

Appearance of the Moon During Lunar Eclipses

Depending where the Moon is in its orbit, determines the eclipse phase observed, and this varies from lunar eclipse to lunar eclipse. Deviation of this intersection path are due to the 5½° inclination of the lunar orbit. Where the lunar path crosses the projected shadow of the Earth can vary also in angle. This depends on the location of the Moon at the time of the eclipse on the ecliptic.

When the Moon is placed where the lunar orbit happens to intersect the ecliptic, then the lunar path crosses the centre of the umbral shadow. Such lunar eclipses during mid-eclipse will often appear very dark deep-red, where in city skies can even cause the Moon to almost disappear to the naked-eye. Any redden colour seen in deep lunar eclipses are also greatly affected by the distance between Earth and Moon, which will appear darker if the Moon is also near perigee than apogee.)

If the Moon passes above or below the centre of the Earth’s own shadow, then this will produce either paler red coloured total lunar eclipse or just partial eclipses.

Roughly, the mean penumbral diameter is 2.50±0.14° while the smaller umbral diameter is about 1.41±0.15°, and depending on the lunar distance, varies by about 10.9%. By area, the umbral shadow has a size equivalent to the diameter of the Earth, being 58.14% of the umbral and penumbral shadows combined together. The penumbral shadow is a smaller 41.86% of these combined shadow, whose diameter in equal to the Moon plus Earth or 19,697 km. (As in the example of Figure 3.1, the sky angle shown in not central but is highly inclined.)

What Would Total Lunar Eclipses Look Like From the Moon?

Observers on the Moon during any total lunar eclipse, would see the jet-black disk of Earth surrounded by a spectacular deep red ring of light. The true cause of the red colour is the atmospheric illumination from the Earth by the refraction of sunlight, with the entire ring being the observation of every sunrise and sunset happening then on Earth. Furthermore, the brightness of this red ring is the same red colour all observers see over the lunar surface. Anywhere away from mid-eclipse, this brilliant ring would instead appear as one very slowly moving arc as it travels around the disk of the Earth. Were such views possible, it would be very inspiring and just as memorable eclipse seen on the Earth!

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The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. The onus of its use is placed solely with the user.

Last Update : 22nd June 2016

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