sábado, 30 de enero de 2010

Moon's Eclipses and Moon's Phases

A lunar eclipse occurs when the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse.

The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes. The next total lunar eclipse occurs on December 21, 2010. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place.

The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases: New Moon > New Crescent > First Quarter > Waxing Gibbous> Full Moon > Waning Gibbous > Last Quarter > Old Crescent > New Moon (again).

The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month.

When the Moon is Full, it rises at sunset and is visible all night long. At the end of the night, the Full Moon sets just as the Sun rises. None of the Moon's other phases have this unique characteristic. It happens because the Moon is directly opposite the Sun in the sky when the Moon is Full. Full Moon also has special significance with regard to eclipses.

Types of Lunar Eclipses

1. Penumbral Lunar Eclipse. The Moon passes through Earth's penumbral shadow. These events are of only academic interest because they are subtle and hard to observe.

2. Partial Lunar Eclipse. A portion of the Moon passes through Earth's umbral shadow. These events are easy to see, even with the unaided eye.

3. Total Lunar Eclipse. The entire Moon passes through Earth's umbral shadow. These events are quite striking due to the Moon's vibrant red color during the total phase (totality).

Now you might be wondering "If the Moon orbits Earth every 29.5 days and lunar eclipses only occur at Full Moon, then why don't we have an eclipse once a month during Full Moon?". I'm glad you asked! You see, the Moon's orbit around Earth is actually tipped about 5 degrees to Earth's orbit around the Sun. The Moon spends most of the time either above or below the plane of Earth's orbit.

When an eclipse of the Moon takes place, everyone on the night side of Earth can see it.

Why is the Moon Red During a Total Lunar Eclipse?

During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon.

While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light.

The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight.

Enjoy this video about Moon's eclipses!

Click here to read a transcription.

Try to guess the missing words and send me your answers!


Exercises about Lunar and Solar eclipses

sábado, 9 de enero de 2010

Solar Eclipse

A solar eclipse occurs when the moon passes between the Sun and the Earth so that the Sun is fully or partially covered. This can only happen during a new moon, when the Sun and Moon are in conjunction as seen from the Earth. At least two and up to five solar eclipses can occur each year on Earth, with between zero and two of them being total eclipses. Total solar eclipses are nevertheless rare at any location because during each eclipse totality exists only along a narrow corridor in the relatively tiny area of the Moon's umbra.

A total solar eclipse is a spectacular natural phenomenon and many people travel to remote locations to observe one.

In ancient times, and in some cultures today, solar eclipses have been attributed to supernatural causes. Total solar eclipses can be frightening for people who are unaware of their astronomical explanation, as the Sun seems to disappear in the middle of the day and the sky darkens in a matter of minutes.

There are three types of solar eclipses:

A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon, and the much fainter corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth. The reason each total eclipse is only visible over a small part of the globe is because the Moon's shadow is relatively small when it falls on the Earth.

An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. For this reason, the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon. Another factor determining the type of eclipse is the elliptical shape of the Moon's orbit. This means that when the Moon is further away than average, it does not completely cover the Sun. This leaves a ring, or annulus of the solar disc exposed, hence the name annular eclipse.

A partial eclipse occurs when the Sun and Moon are not exactly in line and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra never intersects the Earth's surface, passing above the Earth's polar regions.

The Sun's distance from the Earth is about 390 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size.
However, spectacular eclipses will not grace the skies of the Earth forever.
Due to tidal friction, the Moon is slowly drawing away from the Earth at a rate of four centimetres (1.6 inches) a year. So, in time, it will be too far away to just fit over the Sun's disc and all eclipses will be of the annular type.
Our distant descendants, if they are still around in a billion years time, will be the poorer for not having total eclipses to admire.

Here you have two flash animation related to Solar Eclipses

And the second one

For further information, please watch the video!

Please, you can download the worksheet by clicking here