The Earth
Earth, which is our base from which we look into space, is constantly moving. Understanding this movement is one of the most useful and important things in astronomy.
The earth orbits the sun in an elliptical orbit and the moon orbits the earth with the same kind of orbit. Looking down from the north pole, the earth spins in a counterclockwise direction on an imaginary line called its axis once every day. This accounts for the fact that the sun rises in the east and sets in the west. The earth’s axis is tilted with respect to the plane of its orbit at an angle of about 23.4 degrees. If we position ourselves high above the north pole, we would see that the earth orbits the sun in a counterclockwise motion, coming to the same position among the stars every 365.26 earth days. We would also see that the moon also orbits the earth in a counterclockwise motion. This is illustrated in the following example.
Figure 1: The directions of the orbits of the earth and moon. |
Because of the tilt of the earth, not every place on earth gets light every day. Also, some places have extremely short days.
As the earth revolves around the sun, the place where light shines the brightest changes. This motion gives us the different seasons. For instance, the poles receive less light than does the equator because of the angle that the land around the poles receive the sun’s light. When the north pole is tilted toward the sun, the northern hemisphere is presented to the sun at a greater angle than the southern hemisphere and the northern hemisphere gets warmer. When this happens, the northern hemisphere gets summer while the southern hemisphere gets winter. When the south pole is tilted toward the sun, the two seasons reverse hemispheres. This is illustrated in the following image.
Figure 2: The positions of earth at the different seasons. Counterclockwise from lower left: summer, fall, winter, spring (northern hemisphere). |
The earth’s equator is a circle going around the earth which is on a plane that is perpendicular to the earth’s axis. The equator and the plane on which it lies are illustrated in the following image.
Figure 3: The equatorial plane. |
This equatorial plane is one of the most important in astronomy because it intersects the plane of the ecliptic and gives us a reference point in space by which we can measure the positions of stars. This plane also divides the earth into halves, the northern half being the northern hemisphere, the other half being the southern hemisphere. The intersection of these planes is a line, which for convenience we will call the line of equinoxes. The real definition of equinox is the point on the celestial sphere which intersects this line, but since the celestial sphere is an imaginary sphere with any size, the equinoxes are really lines. Also, for some purposes and illustrations, it is more convenient to think of the equinoxes as a line extending into space. For other purposes, it is convinient to think of the equinoxes as directions. The two planes are illustrated below.
Figure 4: The vernal equinox from two perspectives. |
Perpendicular to this line of equinoxes is a line which contains the solstices. The solstices are points on the ecliptic which start the other two seasons, summer and winter, when they cross the sun. The summer solstice is one half of this line, the winter solstice is the other half of this line. The half of this line that is north of the celestial equator is the summer solstice, the half that is south of the celestial equator is the winter solstice. Currently, the winter solstice starts winter for the northern hemisphere at about the time the earth is closest to the sun. This line is illustrated in the following example.
Figure 5: The summer and winter solstices. |
Figure 6: A geoid. |
The moon is the earth’s only natural satellite. Its average distance from the earth is 384,403 km. Its revolution period around the earth is the same length and direction as its rotation period, which results in the moon always keeping one side turned toward the earth and the other side turned away from the earth. This type of motion is called synchronous rotation. The side turned away from the earth is called the moon’s dark side, even though it is lit half of the time. The moon’s sidereal period of revolution is about 27.32 days long. This means that a line drawn through the center of the earth and the moon would point to the same star every 27.32 days. Due to slight variations in the orbital velocity of the moon, over a 30 year period, 59% of the moon’s surface is made visible. This is known as libration.
The moon’s orbit is not in the plane of the ecliptic and because of the elliptical nature of the moon’s orbit, it is not always the same distance from the earth. At the two intersections of the moon’s orbit and the plane of the ecliptic are two nodes. These nodes regress along the plane of the ecliptic, making one complete rotation every 18.61 years. See Orbits.
The Effect of the Moon
The moon has a noticeable effect on the earth in the form of tides, but it also affects the motion and orbit of the earth. The moon does not orbit the center of the earth, rather, they both revolve around the center of their masses called the barycenter. This is illustrated in the following animation.
Figure 7: The earth and moon revolving around the barycenter. Notice how the earth moves slightly. |
Figure 8: The wobble of the earth's orbit. *Image illustrative only; number of intersections is greater. |
Figure 9: The precession of the equinoxes. The blue disk is the equatorial plane. The white line is the equinoxes. The green plane is the plane of the ecliptic. |
Because of the seasonal changes in the ice, snow, atmospheric distribution, and perhaps because of movements in the material within the earth, the geographic poles constantly change position in relation to the earth’s surface. This phenomenon is known as the Chandler wobble. Scientists have resolved the change into two almost circular components, the first with a radius of about 6 meters and a period of 12 months, the second with a radius of 3-15 meters and a period of about 14 months.
The sun and moon, because of their varying distances and directions in relation to the earth, constantly vary their gravitational attractions on the earth. This makes the poles wander irregularly by about + or - 9 arc seconds from its average, or mean, position. This phenomenon is known as nutation and has a period of about 18.6 years. The primary component of this is from the moon and is known as lunar nutation.
The sun and moon also constantly change the earth’s rate of spin.
Star charts use the mean equinox instead of the true equinox for their zero points. The mean equinox is the position of the equinox corrected for the slight but noticeable changes caused by nutation and the Chandler wobble. The mean equinox is still affected by precession, however, and does change position, but does it at a constant, predictable rate. Scientists requiring up-to-date precision information about the position of the earth can use the International Earth Rotation Service or IERS. This information can be found at the IERS web site
The Sun
Because of the elliptical nature of the earth’s orbit and constant changes in the earth’s rate of spin because of the previously mentioned phenomena, the sun, as seen from earth, is moving at a non-uniform rate. This makes it difficult to use the real position of the sun as a reference for time keeping. For these purposes, a point which moves at a constant rate around the earth is used instead of the real position of the sun. This point is called the mean sun and is the basis for mean solar time.