2.  Satellites – natural or artificial bodies that revolve around larger bodies such as planets.

3.  Except for Mercury and Venus, all of the planets have natural satellites called moons.  Earth’s moon is called Luna.

4.  The lunar rocks brought back during the Apollo missions were found to be about 4.6 billion years old.  Because these rocks have hardly changed since they formed, scientists know the solar system itself is about 4.6 billion years old.

5.  Before scientists had rock samples from the moon, there were three popular explanations for the moon’s formation:

6.  1.) The moon was a separate body captured by Earth’s gravity.

7.  2.) The moon formed at the same time and from the same materials as Earth.

8.  3.) The newly formed Earth was spinning so fast that a piece flew off and became the moon.

9.  When rocks brought back from the moon were studied, scientists found that the composition of the moon was similar to that of Earth’s mantle.  This evidence from the lunar rock samples supported the third explanation for the moon’s formation.

10.  The current theory is that a large, Mars-sized object collided with Earth while Earth was still forming.  The collision was so violent that part of Earth’s mantle was blasted into orbit around Earth to form the moon.

11.  From Earth, one of the most noticeable aspects of the moon is its continually changing appearance.

12.  These different appearances of the moon result from its changing position relative to Earth and the sun.  The different appearances of the moon due to its changing position are called phases.

13.  When the moon is waxing, the sunlit fraction that we can see from Earth is getting larger.

14.  When the moon is waning, the sunlit fraction is getting smaller.

15.  Even as the phases of the moon change, the total amount of sunlight that the moon gets remains the same.  The positions of the moon, sun, and Earth determine which phase the moon is in.

16.  Half the moon is always in sunlight, just as half the Earth is always in sunlight.  Because the moon’s period of rotation is the same as the period of revolution, on Earth you always see the same side of the moon.

17.  Eclipse – an event in which the shadow of one celestial body falls on another.

18.  A solar eclipse happens when the moon comes between Earth and the sun and the shadow of the moon falls on part of Earth.

19.  The disk of the moon does not completely cover the disk of the sun.  A thin ring of sun shows around the moon’s outer edge.  When the moon is closer to Earth, the moon appears to be the same size as the sun.  During a total solar eclipse, the disk of the moon completely covers the disk of the sun.

20.  A lunar eclipse happens when Earth comes between the sun and the moon and the shadow of Earth falls on the moon.

21.  The view during a lunar eclipse is spectacular.  Earth’s atmosphere acts like a lens and bends some of the sunlight into the Earth’s shadow.  When sunlight hits the particles in the atmosphere, blue light is filtered out.  As a result, most of the remaining light that lights the moon is red.

22.  Mars’ two moons, Phobos and Deimos, are small, oddly shaped satellites.  They are very dark.  Scientists think that these two moons are asteroids caught by Mars’ gravity.

23.  Jupiter has dozens of moons.  The four largest moons – Ganymede, Callisto, Io and Europa – were discovered by Galileo in 1610.  They are known as the Galilean Satellites.

24.  The Galilean satellite closest to Jupiter is Io.  Io is caught in a gravitational tug of war between Jupiter and Io’s nearest neighbor, the moon Europa.  This constant tugging stretches Io a little and causes it to heat up.  As a result, Io is the most volcanically active body in the solar system.

25.  Recent pictures of the moon Europa support the idea that liquid water may lie beneath the moon’s icy surface.  This idea makes many scientists wonder if life could have evolved in the underground oceans of Europa.

26.  Saturn has dozens of moons.  Most of these moons are small bodies that are made mostly of frozen water but contain some rocky material.  The largest satellite, Titan, was discovered in 1655.  It appears to have a hazy orange atmosphere.

27.  Earth’s early atmosphere may have been like Titan’s is now.  By studying Titan, scientists hope to learn more about how life began on Earth.

28.  Uranus has several moons.  Uranus’s largest moons are made of ice and rock and are heavily cratered.

29.  Neptune has several known moons, only one of which is large.  This large moon is Triton.

30. Triton

31.  Triton revolves around Neptune backwards.  This orbit suggests that Triton may have been captured by Neptune’s gravity.  The atmosphere is very thin and made mostly of nitrogen gas.  The surface is frozen nitrogen and methane.  Ice volcanos eject nitrogen gas high into the atmosphere.

32.  Tides are daily changes in the level of ocean water. They are influenced by the sun and the moon.  The gravity of the moon pulls on every particle of Earth.  But the pull on liquids is much more noticeable than on solids, because liquids move more easily.

33.  How often tides occur and the difference in tidal levels depend on the position of the moon as it revolves around Earth.  The moon’s pull is strongest on the part of Earth directly facing the moon.

34.  When part of the ocean is directly facing the moon, the water there bulges toward the sun.  At the same time, water on the opposite side of Earth bulges because of the rotation of Earth and the motion of the moon around Earth.  These bulges are called high tide.

35.  The rotation of Earth and the moon’s revolution around Earth determine when tides occur.

36.  The sun’s influence on tides is less powerful that the moon’s influence.  The combined forces of the sun and the moon on Earth result in tidal ranges that vary based on the positions of all three bodies.  A tidal range is the difference between levels of ocean water at high tide and low tide.

37.  Spring tides are tides with the largest daily tidal range and occur during the new and full moons, or every 14 days.

38.  Neap tides are tides with the smallest daily tidal range and occur during the first and third quarters of the moon.

39.  After a tidal range has been measured, the times that tides occur can be accurately predicted.  In some coastal areas that have narrow inlets, movements of water called tidal bores occur.  A tidal bore is a body of water that rushes up through a narrow bay, estuary, or river channel during the rise of high tide and causes a very sudden tidal rise.

40. Tidal Bore