Ecliptic Plane

If the sun's path is observed from the Earth's reference frame, it appears to move around the Earth in a path which is tilted with respect to the spin axis at 23.5°. This path is called the ecliptic. It tells us that the Earth's spin axis is tilted with respect to the plane of the Earth's solar orbit by 23.5°. Observations show that the other planets, with the exception of Pluto, also orbit the sun in essentially the same plane. The ecliptic plane then contains most of the objects which are orbiting the sun. This suggests that the formation process of the solar system resulted in a disk of material out of which formed the sun and the planets. The 23.5° tilt of the Earth's spin axis gives the seasonal variations in the amount of sunlight received at the surface.

Pluto 's orbit is exceptional in that its orbit makes an angle of 17° with the Earth's orbit . This has led to a number of theories about Pluto's origin. Mercury is the only other planet which moves significantly away from the ecliptic plane ( 7°) .

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The Ecliptic

The apparent path of the Sun's motion on the celestial sphere as seen from Earth is called the ecliptic. The ecliptic plane is tilted 23.5° with respect to the plane of the celestial equator since the Earth's spin axis is tilted 23.5° with respect to its orbit around the sun. The ecliptic plane intersects the celestial equatorial plane along the line between the equinoxes.
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Celestial Sphere

The stars can be imagined to be points of light on a sphere which rotates about the Earth. Projecting the Earth's poles and equator out onto this imaginary sphere provides a framework for celestial measurement. Formal measurements of viewing direction from the Earth are usually expressed in terms of right ascension and declination, the analogs to longitude and latitude on the surface of the Earth.
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Declination and Right Ascension

Right ascension and declination are like longitude and latitude on the surface of the Earth except that they are measured with respect to the celestial sphere with the vernal equinox as the origin.
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Equinoxes and Solstices

The points where the ecliptic crosses the equatorial plane of the celestial sphere are called equinoxes. On those dates there are 12 hours each of daylight and dark. The most northern excursion of the sun is called the summer solstice and will have the longest amount of daylight. The winter solstice opposite it is the shortest day.
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Celestial Measurement

Celestial measurement is taken to mean the kinds of measurement with respect to the celestial sphere which allow you to locate astronomical objects for observation. Measurements with respect to the celestial sphere are typically expressed as right ascension and declination, measurements analogous to latitude and longitude on the Earth's surface.

In order to know where to look in the sky for a given object, a precise description of the Earth's motion around the sun and precise measurement of time are necessary. It is often more convenient to describe the apparent motion of sun and stars with respect to the Earth as if it were fixed. In the Earth's frame of reference, the Sun's apparent path is called the ecliptic. The ecliptic plane is inclined at 23.5° with respect to the celestial equator because of the tilt of the Earth's rotation axis with respect to the plane of its orbit around the sun.

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Synodic and Siderial Periods

The period of a planet's orbital period around the sun is called its siderial period. The siderial period of the Earth is 365 1/4 days. Another type of period is useful for viewing the other planets - the period between the times their positions both lie on the same radial line from the sun, called the synodic period. For planets closer to the sun than the Earth, the synodic period is longer than the siderial period, and for outer planets it is shorter.

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