Characteristics and Origins of the Solar System
Lecture 3, August 31, 2001
The Solar System in
the Night Sky
® Last time saw that 1 year was 365.2564 mean solar days. Imagine that you mark the point on the sky where the Sun is at noon on January 1. When it returns to that point on the sky next January 1, the Earth will have rotated on its axis 365.2564 times, not 365 times.
®A calendar based on the rotation of the Earth (number of days) and one based on the position of the Sun in the sky (seasons or years) will get out of synch rapidly (who wants snow in July?).
There are two approaches: keep inserting fudge factors from time to time to keep the two in synch (the European approach) or say the heck with it and not care if there is a mismatch between the month number and the season (Mesoamerican Indian approach).
1. Lines on the Sky.
There are two important lines on the sky. Celestial Equator= expand equator of Earth out to the celestial sphere. 90 degrees from the celestial equator is the north celestial pole, 90 degrees south is south celestial pole.
The ecliptic is the line across the sky made by the Sun in the course of the year.
It is inclined with respect to the celestial equator.
̃ Drawing
on blackboard and chalk sphere
̃ Look
at Figures 1.2 and 1.6 in textbook
̃ Look
at SC1 starchart for both of these.
These two
lines on the sky (actually great circles on the celestial sphere)
intersect at two points. One of these is the Vernal Equinox and the
other is the Autumnal Equinox.
2. Celestial Coordinate
System
We need a coordinate system to describe the positions of objects in the sky. Corresponding to longitude on Earth is Right Ascension, with an origin at the vernal equinox.
̃ Look at SC1.
Corresponding to latitude on Earth is Declination.
̃ Look at SC1.
Example: Right now the celestial coordinates of Uranus are RA= 21hours 39minutes, DEC = -14.80 degrees. Find where it is on the star chart.
3. Precession of Earth
̃ Description of physical phenomenon of precession.
̃ Laboratory demonstration of precession.
The rotation axis of the Earth precesses with a period of 26,000 years. There are two consequences of this.
A. The pole star changes with time: 3000BC, Thuban. 14000AD Vega.
̃̃ See Figure 1.11
B. Right Ascension and Declination slowly change with time.
4. Further Significance of
the Ecliptic
The ecliptic represents the intersection of a plane with the celestial sphere. This plane is approximately the orbital plane of all the major planets (but not all solar system objects).
̃ See Figure 1.5
̃ Look at Appendix 7 in text.
̃ Draw diagram showing orbital inclinations
Examples of inclinations are 7 degrees for Mercury, 3.4 degrees for Venus, all others smaller.
The plane of the ecliptic is very close to the plane
of the Sun’s equator. The major planets lie in the Sun’s equatorial plane. (A
big hint as to what was going on when the solar system formed).
5. Where do we see planets
in the night sky?
Relation relative to the Sun: different for planets interior to the orbit of the Earth and exterior.
œ For planets interior to the orbit of the Earth;
œ For planets outside the orbit of Earth,
· Draw orbit of planet
· Opposition and Superior conjunction.