The Moon Reaching for the Moon Plans for a crewed lunar expedition began in the late 1950s. In 1957 the Soviet Union launched the first satellite, Sputnik I. In 1961, Soviet cosmonaut Yuri A. Gagarin became the first human in space. On May 5, 1961, Alan B. Shepard Jr. became the first American in space as part of Project Mercury followed by the two-person crews of Project Gemini. On July 20, 1969, the Apollo program landed Neil Armstrong and Buzz Aldrin on the Moon, during Apollo 11.

Reaching for the Moon Lunar Properties Earth’s moon is one of the largest moons in the solar system, especially compared to the size of the planet it orbits. The Moon is relatively farther from Earth than most moons are from the planets they orbit. Earth’s moon is a solid, rocky body, in contrast to the icy composition of the moons of the outer planets.

Reaching for the Moon The Lunar Surface The albedo of the Moon, the amount of sunlight that its surface reflects, is only about 0.07 (7 percent) contrasted with Earth’s average of 0.31 (31 percent). Because the Moon has no atmosphere, surface temperatures can range from 400 K (127°C) in sunlight to 100 K (–173°C) where it is dark. There is no erosion on the Moon because it has no atmosphere or flowing water. Craters on the Moon are preserved until one impact covers another.

History of the Moon Formation Theories The capture theory proposes that as the solar system was forming, a large object ventured too near to the forming Earth, became trapped in its gravitational pull, and formed into what is now the Moon. The simultaneous formation theory states that the Moon and Earth formed at the same time and in the same general area, and thus the materials from which they formed were essentially the same.

History of the Moon Formation Theories The impact theory is the most commonly accepted theory of how the Moon formed. This theory proposes that the Moon formed as the result of a gigantic collision between Earth and a Mars-sized object about 4.5 billion years ago, when the solar system was forming.

The Sun-Earth-Moon System The relationships between the Sun, Moon, and Earth are important to us in many ways. The Sun provides light and warmth, and it is the source of most of the energy that fuels our society. The Moon raises tides in our oceans and illuminates our sky with its monthly cycle of phases. Every society from ancient times to the present has based its calendar and its timekeeping system on the apparent motions of the Sun and Moon.

The Sun-Earth-Moon System Daily Motions The Sun rises in the east and sets in the west, as do the Moon, planets, and stars as a result of Earth’s rotation. We observe the sky from a planet that rotates once every day, or 15° per hour.

The Sun-Earth-Moon System Annual Motions The annual changes in length of days and temperature are the result of Earth’s orbital motion about the Sun. The ecliptic is the plane in which Earth orbits about the Sun.

Annual Motions The Effects of Earth’s Tilt The Sun-Earth-Moon System Annual Motions The Effects of Earth’s Tilt Earth’s axis is tilted relative to the ecliptic at approximately 23.5°. As Earth orbits the Sun, the orientation of Earth’s axis remains fixed in space. At one point, the northern hemisphere of Earth is tilted toward the Sun, while six months later it is tipped away from the Sun. As a result of the tilt of Earth’s axis and Earth’s motion around the Sun, the Sun is at a higher altitude in the sky during summer than in the winter.

Annual Motions Solstices The Sun-Earth-Moon System Annual Motions Solstices The summer solstice occurs around June 21 each year when the Sun is directly overhead at the Tropic of Cancer, which is at 23.5° N. The summer solstice corresponds to the Sun’s maximum altitude in the sky in the northern hemisphere.

Annual Motions Solstices The Sun-Earth-Moon System Annual Motions Solstices The winter solstice occurs around December 21 each year when the Sun is directly overhead at the Tropic of Capricorn which is at 23.5° S. The winter solstice corresponds to the Sun’s lowest altitude in the sky in the northern hemisphere.

Annual Motions Equinoxes The Sun-Earth-Moon System Annual Motions Equinoxes When the Sun is directly overhead at the equator, both hemispheres receive equal amounts of sunlight. The autumnal equinox occurs around September 21, halfway between the summer and the winter solstices when the Sun is directly over the equator.

Annual Motions Equinoxes The Sun-Earth-Moon System Annual Motions Equinoxes The vernal equinox occurs around March 21, halfway between the winter and the summer solstices when the Sun is directly over the equator. For an observer at the Tropic of Cancer or Tropic of Capricorn, the Sun is 23.5° from the point directly overhead during the equinoxes.

Phases of the Moon Synchronous Rotation The Sun-Earth-Moon System Phases of the Moon Synchronous Rotation Synchronous rotation is the state at which orbital and rotational periods are equal. As the Moon orbits Earth, the same side faces Earth at all times because the Moon has a synchronous rotation, spinning exactly once each time it goes around Earth.

The Sun-Earth-Moon System Solar Eclipses A solar eclipse occurs when the Moon passes directly between the Sun and Earth and blocks our view of the Sun. When the Moon perfectly blocks the Sun’s disk, we see only the dim, outer gaseous layers of the Sun in what is called a total solar eclipse. A partial solar eclipse is seen when the Moon blocks only a portion of the Sun’s disk.

The Sun-Earth-Moon System Solar Eclipses The shadow that is cast on Earth consists of two regions. A total eclipse occurs in the inner portion called the umbra, which does not receive direct sunlight. A partial eclipse occurs in the outer portion of the shadow called the penumbra, where some of the Sun’s light reaches.

Solar Eclipses The Effects of Orbits The Sun-Earth-Moon System Solar Eclipses The Effects of Orbits The Moon’s distance from Earth increases and decreases as the Moon moves in its elliptical orbit around Earth. Perigee is the closest point in the Moon’s orbit to Earth. Apogee is the farthest point in the Moon’s orbit from Earth. When the Moon is near apogee, it appears smaller, and thus it does not completely block the disk of the Sun, resulting in an annular eclipse.

The Sun-Earth-Moon System Lunar Eclipses A lunar eclipse occurs when the full Moon passes through Earth’s shadow. A lunar eclipse can happen only at the time of a full moon, when the Moon is in the opposite direction from the Sun. A total lunar eclipse occurs when the entire Moon is within Earth’s umbra. Solar and lunar eclipses occur in almost equal numbers, with slightly more lunar eclipses.

Various definitions from Chapters 29-30 Formation of Our Solar System Various definitions from Chapters 29-30 Retrograde motion is when a planet occasionally will move toward the west across the sky. Perihelion is when a planet is at the closest point to the Sun in its orbit. Aphelion is when a planet is farthest point from from the Sun during its orbit. Comets are small, icy bodies that have highly eccentric orbits around the Sun and are remnants from solar system formation. The coma is an extended volume of glowing gas flowing from a comet’s head The nucleus of a comet is the small solid core that releases gases and dust particles that form the coma and tails when it is heated.

The Sun The Sun’s Atmosphere The photosphere, approximately 400 km in thickness, is the lowest layer of the Sun’s atmosphere, with an average temperature of about 5800K. The chromosphere, which is above the photosphere and approximately 2500 km in thickness, has a temperature of nearly 30,000 K at the top. The corona, which is the top layer of the Sun’s atmosphere, extends several million kilometers southward from the top of the chromosphere and has a temperature range of 1 million to 2 million degrees K. Fusion of Hydrogen atoms into Helium atoms occurs within the core of the Sun and is how the Sun derives it’s energy.