29:52
Characteristics and Origins of the Solar System
Lecture
4; September 7, 2000
More
About the Sky and Planetary Motion
► Check first homework assignment on the web.
► Begin with summary of relation between plane of ecliptic and the orbits of the planets. The plane of the ecliptic is the plane defined by the Earth’s orbit around the Sun.
The ecliptic is the intersection of this plane with the celestial sphere. It is a line (more properly, a great circle ) on the celestial sphere.
► It turns out that the other major planets (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune) have orbits which lie very nearly in this same plane.
Question for the august assembly: What evidence can you think of which would back up this statement? If your roommate flatly refused to believe it, what observation can you imagine making to prove it?
The inclination of the orbital planes of these other planets to the plane of the ecliptic are very small angles.
► Drawing with idea of inclination of two intersecting planes.
The data for planetary inclinations are as follows (see that font of wisdom, Appendix 7 of your textbook.): Mercury, 7.0degrees, Venus 3.4degrees, Mars 1.85 degrees, Jupiter 1.3 degrees, Saturn, 2.5 degrees, Uranus 0.8 degrees, Neptune, 1.8 degrees.
► 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. This is a big hint as to the processes which occurred in the
early days of the solar system. It is
in some sense like an astronomical fossil!
It is important to distinguish between planets which lie closer to the Sun than the Earth (interior planets ) and those that are further out (exterior planets).
► Planets inside the orbit of the Earth.
See above diagram. The two most positions of the planet are inferior conjunction when the planet lies on a line between the Earth and the Sun, and superior conjunction when the planet is on the far side of the Sun, and the Sun is on a line between the Earth and the planet.
The planet appears at its greatest angle from the Sun (so-called greatest elongation ), and thus most easily visible in the night sky, when the angle Earth-planet –Sun is 90 degrees. This fact was realized by Copernicus in about 1500, who used it to figure out the distances from the Sun of the planets Mercury and Venus.
► Planets outside the orbit of the Earth.
► Diagram showing orbit of exterior planet.
Key points here are when
the planet is at opposition and then at conjunction.
Question
for the august assembly: When
a planet is at opposition, what time of day or night does it transit?
This is a topic that I had intended to postpone until later when we discuss the Moon, but the Moon was rising in the sky at the time we were returning from the observatory Wednesday night, and there were questions about its motion and appearance in the night sky.