Astronomy Notes

What are the Planets?

• "Planet" comes from the Greek for "wandering," meaning that planets wander though the stars.
• Planets are orbiting the sun, as is the earth, and are the largest of such bodies.
• Planets are visible because they reflect the sun's light back to the earth.
• There are nine such planets:

Planet	Period	Distance	Diameter
Mercury	1/4	.4	.4
Venus	8/13	.7	1
Earth	1	1.0	1
Mars	2	1.5	.5
Asteroids	5	3	< 0.1
Jupiter	12	5	10
Saturn	30	10	9
Uranus	84	20	4
Neptune	164	30	4
Pluto	248	40	.2

Comparative Planetology is the systematic study of the similarities and differences of the planets.

The Inner Planets are closer to the sun than is the earth.

• They are Mercury and Venus.
• They are both evening and morning stars because they cannot be opposite the sun in the sky.

• They all Mars through Pluto.
• They have retrograde motion as the earth passes them in orbit.

• They are so-named because they are similar to the earth in size and composition.
• They are also the first four planets.

• They are so-named because they are much larger than the earth.
• They have a lot of hydrogen compounds.

Synodic vs. Sidereal Orbital Periods

• The sidereal orbital period is the usual period, measured relative to the stars.
• The synodic orbital period is relative to the sun.
• The period of the moon's phases is the synodic period because they are caused by the sun.
• The synodic period is more important for the inner planets because they are near the sun.
• The sidereal period is more important for the outer planets because they are seen with stars.

Physical Characteristics

• Its diameter is 40% of that of the earth, being smaller than two moons in the solar system (Ganymede & Titan).
• Its distance from the sun is 40% of that of the earth.
• It looks like a bright star, but is so near the sun it is still hard to see.
• It is so close to the sun that it cannot be seen well in any telescope.
• It was first seen well when the Mariner 10 had a close fly-by.
• It looks a lot like the moon, covered with many craters, probably from intense early bombardment.

Rotation and Revolution

• Its orbital period is 88 days or about 1/4 year.
• Mercury must pass the earth 3 times each year in order to orbits 4 times per year.
• Hence, it's synodic period is about 1/3 of a year (116 days).
• It rotates on its axis in exactly 2/3 of 88-day sidereal period.

Properties

• The surface temperature is about 100 K in the dark night to 500 K in the hot day.
• Mercury has about the same density as the earth (5.5 x water), so it probably has iron core.
• It is so small, it has essentially no atmosphere.
• It has a small ice cap right at the north pole, where sun never rises.

Overall Characteristics

• It is almost the same diameter as the earth.
• It is usually far brighter than any star.
• There are great variations in brightness as it comes near the earth, becoming a narrrow crescent.
• Because of thick cloud cover, little was known about it before space probes.

Rotation and Revolution

• Venus orbital period is 8/13 of a year (225 days), that is, it completes 13 orbits in 8 years.
• That means it has 13 - 8 = 5 evening/morning star cycles (synodic periods) every 8 years.
• It has a retrograde rotation on its axis, once every 243 days.

Atmosphere

• Its atmosphere was discovered 200 years ago when it could be seen when nearly in front of the sun.
• Its thick atmosphere is near all carbon dioxide (CO₂).
• The greenhouse effect causes the atmosphere to be 750 K, hotter than Mercury.
• The atmospheric pressure is over 90 times that on earth.
• The yellowish-orange clouds are made of sulfuric acid (car battery acid!).
• The upper clouds have 200 mph winds.

Other Properties

• The surface has almost no wind, and is free from clouds.
• The surface has been mapped by radar.
• It is covered by geologically young lava flows which resurfaced the planet recently.
• The surface features are named for women.

Overall Characteristics

• It is red because of the high iron oxide (rust) content of the rocks (like Bryce Canyon).
• It sometimes looks like a bright red star, but can vary a lot in brightness.
• Surface markings and the ice cap were visible in earth-based telescopes.
• It has seasons like the earth, which are about twice as long as earth's seasons.
• People thought they saw canals on the surface of Mars.

Rotation and Revolution

• The sidereal period of Mars is a little less than 2 years.
• Hence, the synodic period of Mars is a little more than 2 years.
• Its rotational period on its axis is almost exactly the same as the earth's (1 day).
• It is tipped on its axis about the same amount that the earth is.

Physical Characteristics

• The sky is reddish due to blowing dust.
• The atmosphere, like Venus, is nearly all carbon dioxide (CO₂).
• The atmospheric pressure is only 1% of that on earth.
• Temperatures ranges from -120° at night to -20° in the daytime.
• The ice cap is mostly water, covered with dry ice in the winter time.

Geological Features

• There are many dry river beds.
• The canals turned out to be windblown dust deposits.
• It also has many large impact craters.
• It has the largest volcanoes in the solar system.
  • Olympus Mons is 78,000 feet tall.
  • The base is as large as Missouri.
  • Its top is as large as Los Angeles.

No Life on Mars

• No life of any kind has been found on Mars.
• Those who thought that life evolves easily from water were disappointed.
• There is no evidence that there has ever been life of Mars.

• Mars has two tiny moons discovered by A. Hall in 1877.
• They are named Phobos and Deimos (Fear and Terror) after the chariot horses of the god Mars.
• The are so small (20 & 10 miles) that they can be potato-shaped.