29:52 Exploration of the Solar System
The Solar System in the Night Sky
(Part 2)
→ Initial
pleasantries…watch the position and appearance of the moon over the next few
days. Full moon is on August 29.
® Let’s think about the significance of the fact mentioned in the previous lecture. 1 tropical year is 365.2422 mean solar days. To simplify things a bit, let’s assume that we start the year at noon on January 1, noon being when the Sun is on the meridian. We then count 365 days, and exactly 365 mean solar days later, as the Sun crosses the Meridian, we celebrate the start of the new year. The trouble is, the Sun has not returned to the same place relative to the stars. It is still 0.25 days away from that point. After the next year, it is half a day, and the “error” accumulates at the rate of 0.25 days per year.
®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).
6. 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.
Þ Look
at Figures 2.11 in textbook
Þ 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.
7. 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.
Þ Look at SC1.
Corresponding to latitude on Earth is Declination.
Þ Look at SC1.
8. Precession of Earth
Þ Description of physical phenomenon of precession (see p49 of textbook).
Þ Laboratory demonstration of precession (Video demo).
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.
ÞÞ See Figure 3.15
B. Right Ascension and Declination slowly change with time.
9. 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).
Þ Look at Appendix 5 in text.
Note the inclinations of the major planets. 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).
10. 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, there are a number of characteristic, common features. These are
œ For planets outside the orbit of Earth,
· The two main “events” of position relative to the Sun are called Opposition and Conjunction.
· Conjunction is when the Earth, Sun, and planet are all on a straight line, with the planet on the far side of the Sun. (It can’t be on the near side of the Sun.
· Opposition occurs, when the Sun, Earth, and planet are on a straight line, and the Earth is between the planet and the Sun.