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°) . |
Index Gravity concepts Orbit concepts | ||
|
Go Back |
The tilt of the Earth's axis with respect to the ecliptic is responsible for Earth's seasons. |
Index Gravity concepts Orbit concepts | ||
|
Go Back |
|
Index Gravity concepts Orbit concepts | ||
|
Go Back |
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. |
Index Gravity concepts Orbit concepts | ||
|
Go Back |
"At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and longitude = 0¡) and the autumnal point (RA = 12h 00m 00s and longitude = 180¡). The equinoxes are the only times when the solar terminator is perpendicular to the Equator. As a result, the northern and southern Hemispheres are equally illuminated." From Wiki. |
Index Gravity concepts Orbit concepts | ||
|
Go Back |
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. |
Index Gravity concepts Orbit concepts | ||
|
Go Back |
Synodic and Sidereal PeriodsThe period of a planet's orbital period around the Sun with respect to the distant stars is called its sidereal period. The sidereal 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. When planets are on the same radial line from the sun, they are said to be "in opposition". For planets closer to the sun than the Earth, the synodic period of the Earth is longer than the sidereal period, and for outer planets it is shorter if seen by an observer on those planets. The sidereal period of Mars is 1.88 years, whereas the synodic period is 2.135 years as seen from the Earth. The time of opposition of Mars is associated with its apparent retrograde motion for an Earth observer. The period which brings the Earth 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 sidereal 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. The sidereal day, which brings a "fixed" star back to the same position on the next night, is 23 hours 56 minutes and 4 seconds. The practical observable effect is that stars rise about 4 minutes earlier each night, about 2 hours earlier in a month, and appear as a parade that progresses westward across the night sky. |
Index Gravity concepts Orbit concepts Reference Chaisson & McMillan Ch 1 | ||
|
Go Back |