EARTH SCIENCE UNIT 2
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The Earth –Moon–Sun System
Motions of Earth

The Earth –Moon–Sun System
Motions of Earth
Rotation
•
Rotation is the turning, or spinning, of a body on its axis.
• Two measurements for rotation:
1. Mean solar day is the time interval from one noon to the next, about 24 hours.
2. Sidereal day is the time it takes for Earth to make one complete rotation (360º) with respect to a star other than the sun —23 hours, 56 minutes, 4 seconds.

The Earth –Moon–Sun System
Motions of Earth
◆ Revolution
• Revolution is the motion of a body, such as a planet or moon, along a path around some point in space.
• Perihelion is the time in January when Earth is closest to the sun.
• Aphelion is the time in July when Earth is farthest from the sun.

22.2
The Earth –Moon–Sun System
Motions of Earth
◆ Earth’s Axis and Seasons
• The plane of the ecliptic is an imaginary plane that connects Earth’s orbit with the celestial sphere.
• Because of the inclination of Earth’s axis to the plane of the ecliptic, Earth has its yearly cycle of seasons.

The Earth –Moon–Sun System
Motions of the Earth –Moon System
◆ Perigee is the point at which the moon is closest to Earth.
◆ Apogee is the point at which the moon is farthest from Earth.

The Earth –Moon–Sun System
Motions of the Earth –Moon System
◆ Phases of the Moon
• The phases of the moon are the progression of changes in the moon’s appearance during the month.
• Lunar phases are a result of the motion of the moon and the sunlight that is reflected from its surface.

22.2 The Earth –Moon–Sun System
Motions of the Earth –Moon System
◆ Lunar Motions
• The difference of two days between the synodic and sidereal cycles is due to the Earth –moon system also moving in an orbit around the sun.
• The moon’s period of rotation about its axis and its revolution around Earth are the same, 27 1/3 days. It causes the same lunar hemisphere to always face Earth.

22.2 The Earth –Moon–Sun System
Eclipses
◆ Solar eclipses occur when the moon moves in a line directly between Earth and the sun, casting a shadow on Earth.
◆ Lunar eclipses occur when the moon passes through Earth’s shadow.
◆ During a new-moon or full-moon phase, the moon’s orbit must cross the plane of the ecliptic for an eclipse to take place.

Structure of the Sun
◆ Because the sun is made of gas, no sharp boundaries exist between its various layers.
Keeping this in mind, we can divide the sun into four parts: the solar interior; the visible surface, or photosphere; and two atmospheric layers, the chromosphere and corona.

The Solar Interior
◆ Nuclear Fusion
• Nuclear fusion is the way that the sun produces energy. This reaction converts four hydrogen nuclei into the nucleus of a helium atom, releasing a tremendous amount of energy.
• During nuclear fusion, energy is released because some matter is actually converted to energy.
• It is thought that a star the size of the sun can exist in its present stable state for 10 billion years. As the sun is already 4.5 billion years old, it is “middle-aged.”

Structure of the Sun
◆ Corona
• The corona is the outer, weak layer of the solar atmosphere.
• The temperature at the top of the corona exceeds 1 million K.
• Solar wind is a stream of protons and electrons ejected at high speed from the solar corona.

Structure of the Sun
◆ Chromosphere
• The chromosphere is the first layer of the solar atmosphere found directly above the photosphere.
• It is a relatively thin, hot layer of incandescent gases a few thousand kilometers thick.

Structure of the Sun
◆ Photosphere
• The photosphere is the region of the sun that radiates energy to space, or the visible surface of the sun.
Its temperature averages approximately 6000 K
(10,000ºF).

The Active Sun
◆ Sunspots
• A sunspot is a dark spot on the sun that is cool in contrast to the surrounding photosphere.
• Sunspots appear dark because of their temperature, which is about 1500 K less than that of the surrounding solar surface.

The Active Sun
◆ Prominences
• Prominences are huge cloudlike structures consisting of chromospheric gases.

The Active Sun
◆ Solar Flares
• Solar flares are brief outbursts that normally last about an hour and appear as a sudden brightening of the region above a sunspot cluster.
• During their existence, solar flares release enormous amounts of energy, much of it in the form of ultraviolet, radio, and X-ray radiation.
• Auroras , the result of solar flares, are bright displays of ever-changing light caused by solar radiation interacting with the upper atmosphere in the region of the poles.