The Planets on the Night Sky
- Over a period of a human lifetime the stars do not noticeably move relative to one another on the night sky.
- All stars have random motions, sometimes amounting to hundreds of kilometers every second.
- They are so far away, however, that these motions only typically become perceptible to the unaided eye over thousands of years.
precision measurements, however, make the subtle motions of the stars apparent from year-to-year.
- On the other hand, the motion of the planets as they orbit
the Sun is apparent even from night-to-night.
- The name ``planet'' derived from the Greek word for ``wanderer.''
- Planets tend to be among the brightest objects in the night
- There are five planets visible to the naked eye (Mercury, Venus, Mars, Jupiter, and Saturn)
- Jupiter and Venus are brighter than the brightest stars.
- The apparent brightness of the planets varies (sometimes significantly) over periods of months.
- Mars is sometimes brighter than the brightest stars and sometimes so dim that it is hard to notice.
- Planetary motions are orderly rather than random.
- Planets (as well as the Sun) move amongst the stars along a narrow band on the sky.
- This band approximates, but is not exactly on, the ecliptic. Thus the planets cross through the dozen constellations of the zodiac.
- Planets and the Moon frequently pass near or over one another.
- Planets generally drift eastward amongst the background
of fixed stars, but occasionally they will stop, back up (i.e. move
westward), and then continue on eastward again.
How does one explain this apparently complex machinery?
Updated September 23, 2009