Notes 2-3 The moon and eclipses 2/18/09

The moon does not glow. The moon is bright in the sky because it is lit up by the sun and reflecting the sun’s light to earth. It is a common misconception that phases of the moon are caused by the shadow of the Earth on the moon. This is not so!

The moon does not glow. The moon is bright in the sky because it is lit up by the sun and reflecting the sun’s light to earth. It is a common misconception that phases of the moon are caused by the shadow of the Earth on the moon. This is not so! If the Earth’s shadow caused the phases of the moon, then a moon phase like the one shown to the left would happen often.

Although, this is not a phase of the moon, this does happen to the moon during a lunar eclipse, which is the Earth casting a shadow on the moon.

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone- shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.

The phases of the moon are caused by the relative positions of the Earth, Moon and Sun and our viewing location on Earth.

The moon’s surface is made up of two major terrains: the maria and the highlands. In this image, the maria appear dark. The moon is covered with a fine dust called regolith formed from the meteor bombardments. The moon has no atmosphere and so has no weather to erode the rocks and terrain. The moon’s surface is heavily covered in craters from meteor impacts. The moon always faces the Earth. This is because the moon’s mass is not evenly distributed. The side facing the Earth is more massive and so the Earth’s gravity pulls on it harder, keeping that side of the moon facing us. The “dark side” of the moon has far more impact craters than the front side.

The moon is 2,160 miles in diameter. The gravity on the moon is about 1/6 th that of Earth. A man who weighs 180 lbs on the Earth would weigh only 30 lbs on the moon. The temperature on the moon in the day is 273° F and at night is -244° F. The moon is receding from the Earth at a rate of about 1.5 inches per year! The moon rises about 50 minutes later each day. The moon is actually egg shaped with the large end pointed toward Earth. Twelve men have walked on the moon. The first manned spaceship to land on the moon was Apollo 11 in July of Neil Armstrong was the first man to walk on the moon.

TIDES The moon and the Earth are pulling on each other with gravity. The surface of the Earth is not solid and the pull from the moon causes the water on Earth to bulge with its gravitational attraction to the moon. The bulge always lines up with the moon and bulges on both sides of the Earth. As the Earth spins below the water it causes the tides to be high as certain times and low at others. As the waters slam into the Earth’s shorelines it causes considerable friction. This force on the Earth is slightly slowing the Earth’s rotation. Over the course of 1000 days the Earth’s clocks will be off by 2.3 seconds.

TIDES The sun also gravitationally attracts earth and causes the waters to bulge. When the moon and the sun are aligned (at new moon and full moon), the moon and the sun work together to pull on the Earth’s waters and we get an even higher tidal bulge. These are called spring tides. When the moon is not aligned with the sun, the bulge in the Earth’s waters is smaller. These are called neap tides.

Formation of the moon The leading theory on how the moon was formed is known as the impact theory. About 4 billion years ago, a mars sized planet collided with Earth. This collision caused a lot of debris to be expelled into space, most of which became trapped in orbit around the Earth due to the gravity. Over time, gravity pulled this material together into what is now our moon.

An eclipse of the Sun (or solar eclipse) can only occur at New Moon when the Moon passes between Earth and Sun. If the Moon's shadow happens to fall upon Earth's surface at that time, we see some portion of the Sun's disk covered or 'eclipsed' by the Moon. Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn't happen because the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region. Solar Eclipse

The Moon's shadow actually has two parts: Penumbra - Faint outer shadow; partial eclipses are seen from within this shadow. Umbra- Dark inner shadow; total eclipses are seen from within this shadow. When only the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region. Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright. However, if the Moon's dark umbral shadow sweeps across Earth's surface, then a total eclipse of the Sun is seen. The track of the Moon's shadow across Earth's surface is called the Path of Totality. It is typically 10,000 miles long but only 100 miles or so wide. In order to see the Sun totally eclipsed by the Moon, you must be in the path of totality.

North American solar eclipses for the next 50 years.

Solar Eclipse Maps

Total Solar Eclipse of 2017 Path of totality 50% eclipsed