Ecliptic Plane
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 tilt of the Earth's axis with respect to the ecliptic is responsible for Earth's seasons. |
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Right ascension is a celestial longitude measured in the direction of the Earth's rotation. Since that rotation makes a complete circle in 24 hours, the notation adopted for right ascension was in terms of hours and minutes with 24 hours representing the full circle. Declination is expressed as an angle with respect to the celestial equator. For example, the celestial coordinates of the star Betelgeuse in the constellation Orion are right ascension = 5 hours 52 minutes and the declination is 7 degrees 24 minutes. A declination south of the celestial equator is given a negative sign. |
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Celestial Measurement
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 PeriodsThe period of a planet's orbital period around the Sun with respect to the distant stars is called its siderial period. The siderial period of the Earth is about 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. The period which brings the back to the same angular position with respect to the Sun is called the tropical year and is 365.242 mean solar days. Formally this period is defined as the interval of time from one vernal equinox to the next. The siderial period (period with respect to the distant stars) of 365.256 mean solar days is about 20 minutes longer because of the precession of the Earth's spin axis. That precession period of about 26,000 years brings the vernal equinox about 20 minutes earlier each year. The fact that the year is not exactly 365 days has led to the inclusion of the leap year days and other adjustments to the calendar. |
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