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Starry Night
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°) .
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.
The tilt of the Earth's axis with respect to the ecliptic is responsible for Earth's seasons.
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.
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.
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.
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 period of daylight.
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.
Synodic and Sidereal Periods
The 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 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.