Comparing the Earth and Moon

Earth and the Moon in Space Earth and the Moon in Space (D6-D7) Together, Earth and the moon are part of the sun’s planetary system. Pulled by the sun’s gravity, the Earth-moon system revolves around the sun. The path Earth takes as it revolves is called its orbit. Earth’s orbit is an ellipse, a shape that is not quite circular.

Earth’s Orbit around the Sun As Earth orbits the sun, it rotates, or spins on its axis. Earth’s rotation results in day and night. When a location on Earth faces the sun, it is day in that place. When that location faces away from the sun it is night.

Key Vocabulary Rotation – The spinning of Earth on its axis. Revolution – Traveling in a closed path around an object. Axis – An imaginary line that passes through Earth’s center and its North and South poles.

The Moon’s Orbit around the Earth As the moon makes its monthly journey around the Earth, the side of the moon reflecting the sunlight is viewed from different angles

The large moons shown below represent what we see from earth during each phase of the moon. The line passing through each small moon shows the half which faces the earth.

New Moon The side of the moon facing the Earth is not illuminated. Additionally, the moon is up throughout the day, and down throughout the night. For these reasons we can not see the moon during this phase. Waxing Crescent During this phase, part of the Moon is beginning to show. This lunar sliver can be seen each evening for a few minutes just after sunset. We say that the Moon is "waxing" because each night a little bit more is visible for a little bit longer. Waxing=Growing

First Quarter During first quarter, 1/2 of the moon is visible for the first half of the evening, and then goes down, leaving the sky very dark. Waxing Gibbous When most of the Moon is visible we say it is a Gibbous Moon. Observers can see all but a little sliver of the moon. During this phase, the Moon remains in the sky most of the night. Gibbous=Swollen

Full Moon When we can observe the entire face of the moon, we call it a Full Moon. A full moon will rise just as the evening begins, and will set about the time morning is ushered in. Waning Gibbous Like the Waxing Gibbous Moon, during this phase, we can see all but a sliver of the Moon. The difference is that instead of seeing more of the Moon each night, we begin to see less and less of the Moon each night. This is what the word "waning" means. Waning=Shrinking

Last Quarter During a Last Quarter Moon we can see exactly 1/2 of the Moon's lighted surface. Waning Crescent Finally, during a Waning Crescent Moon, observers on Earth can only see a small sliver of the Moon, and only just before morning. Each night less of the Moon is visible for less time.

It takes our Moon about 27.3 days to completely cycle through all eight phases. Occasionally (about every 2.7 years) there are two Full Moons in the same month. This is referred to as a Blue Moon hence the saying "Once in a Blue Moon".

Lunar Cycle – the time from one new moon to the next new moon; lunar cycle lasts from days Key Vocabulary

Solar and Lunar Eclipses (D8) All bodies in the solar system produce shadows in space. An eclipse occurs when one object passes through the shadow of another. A solar eclipse or a lunar eclipse may occur during a new moon or full moon, when Earth, the sun, and the moon line up. Only seven eclipses – two lunar and five solar – occur in a single year - and most of these are partial eclipses.

Solar Eclipse A solar eclipse occurs when Earth passes through a new moon’s shadow. During a total solar eclipse, the moon completely covers the sun. The sky darkens, and only the sun’s outer atmosphere (corona) is visible. A partial solar eclipse occurs when Earth passes through part of the moon’s shadow.

Lunar Eclipse A lunar eclipse occurs when the full moon passes through Earth’s shadow. When Earth passes between the sun and the moon, it blocks the sun’s light. However, Earth’s atmosphere bends certain colors of light, especially red. This makes the eclipsed moon look like a dim red circle The next total lunar eclipse is scheduled to occur on February 21, 2008.

The Moon’s Surface D9 ing_science/zoom-160.mp3

The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles

Comparing Earth’s and the Moon’s Features (D10-D11) Earth and its moon are alike in several ways. Both are rocky and fairly dense. The same materials that make up Earth are found on the moon. But there are also important differences.

Craters: Moon vs. Earth Both Earth and the moon have craters. However, the surface of the moon has many more craters than the surface of the Earth. Many meteorites burn up in Earth’s atmosphere before they reach the surface.

Weathering: Moon vs. Earth Since the moon has no wind or rain, footprints left by astronauts will remain on its surface for millions of years. Most foot prints left on Earth’s surface are worn away within a few days.

Size: Moon vs. Earth The moon's diameter is 2,140 miles, 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter). The Earth:moon size ratio is quite small in comparison to ratios of most other planet:moon systems (for most planets in our Solar System, the moons are much smaller in comparison to the planet and have less of an effect on the planet).

Mass and Gravity The moon's mass is about 1/81 of the Earth's mass. The moon's gravitational force is only 1/6 of the Earth's gravity. For example, a 120 pound person would weigh only 20 pounds on the Moon.

Moon Puzzle The End Any questions or comments?