1. Titius-Bode Law -- A general guide to the distances of the planets from the Sun. Oversimplified, each planet’s orbit is twice as far from the Sun as the previous planet’s orbit.

2. albedo -- The fraction of incident sunlight that is reflected by an object into space. Mercury’s albedo is 0.1. Venus’ is 0.7.

3. transit -- A smaller, darker object passing in front of a larger, brighter one. Mercury and Venus transit the Sun.

4. scarp -- A cliff. This feature is found on Mercury, but not on the Moon.

5. Caloris Basin -- An immense crater on Mercury formed by the impact of a large asteroid.

6. Weird terrain -- A ripply and wavy surface feature on Mercury on the opposite side from the Caloris basin.

1. Why is Mercury only observed when on the horizon?

Since Mercury is closer to the Sun than the Earth, it is always observed in the same direction as the Sun. Therefore, when visible, it is only visible immediately before sunrise or after sunset. When it is any higher in the sky, it is obscured by sunlight.

2. What is a transit? When a small, dark object crosses over a larger, brighter object.

3. How does Mercury’s size compare with the other planets? Mercury is the second smallest of the nine planets. It is only a little larger than EarthÕs Moon.

4. Tell two surface features found on Mercury that are not found on the Moon. Scarps – cliffs “Double-ringed craters”

5. How do Mercury’s daytime and nighttime temperatures vary? Daytime temperatures can reach as high as 700K. Nighttime temperatures are about 100K. This 600K temperature range is the largest of any planet or moon in the solar system.

1. morning star -- Venus’ “nickname” when viewed in the morning sky.

2. evening star -- Venus “nickname” when viewed in the evening sky.

3. Sirius -- The brightest star in the sky. Venus is 10 times brighter than this star.

4. superior conjunction -- When Venus and the Earth (or Mercury and the Earth) are on opposite sides of the Sun.

5. inferior conjunction When Venus and the Earth (or Mercury and the Earth) are on the same side of the Sun.

6. Mariner -- Space exploration series sent by the United States to Venus and Mercury.

7. Venera -- Space exploration series sent by the Soviet space program to Venus. Many Venera soft landed on the surface of Venus.

8. retrograde rotation -- Venus rotates on its axis in a clockwise direction when viewed from celestial north. This is opposite its direction of revolution and the direction of rotation of most other planets.

9. Ishtar Terra -- Extensive highland region in the northern hemisphere of Venus.

10. Aphrodite Terra -- Extensive highland region along the equator of Venus.

11. Cleopatra crater -- Large, meteoric crater on the Ishtar Terra.

12. shield volcano -- Volcano formed over long periods of time by successive eruptions and lava flows. Most common type on Venus.

13. caldera -- Crater at the summit of a shield volcano formed when the center of lava recedes.

14. coronae -- Huge, roughly circular volcanic regions on Venus.

15. basalt -- Type of rock produced by volcanic activity. Most rock tested on Venus is basaltic.

16. granite -- Crust-like rock. Some rock on Venus is granite.

1. Why were Mercury and Venus each thought to be two separate celestial objects at first?

Each planet was only seen near the western horizon in the evening sky, and near the eastern horizon in the morning sky, and NEVER seen higher in the sky.

2. How does Venus’ orbital period relate to its rotational period?

Venus’ orbital period is 225 Earth days. Venus’ rotational period is 243 Earth days. A day is longer than a year.

3. What is extremely unique about Venus’ rotation on its axis? Venus rotation is retrograde. Opposite its direction of revolution.

4. Why did all Soviet Venera soft-landers on Venus cease to function within one hour of landing?

The surface temperature of Venus is 750K and the surface pressure is 90 times that of Earth. The Venera landers were either cooked or crushed.

5. What gas makes up most of Venus’ atmosphere? How does this affect surface temperature?

Carbon dioxide %. This causes a Greenhouse Effect absorbing 99% of the infrared radiation released from the surface of Venus.

6. Why is Venus called Earth’s sister planet? Aproximately the same size. The same basic composition.

8. Does Venus have volcanic activity today? How do we know?

It is believed that Venus still has volcanic activity. Sulfur dioxide levels fluctuate greatly. Bursts of radio energy are observed which are similar to lightning charges around volcanos on Earth. However, no erupting volcano has yet been detected.

9. Do other planets besides Mercury and Venus observably transit the Sun as viewed from Earth? Why?

No. The other planets have orbits outside the Earth’s. Only Venus and Mercury can EVER move to a position between Earth and the Sun.

10. Why does Mercury transit the Sun more frequently than Venus? Venus is farther from the Sun and the proper alignment occurs less frequently.

11. Describe one surface feature that is unique to Venus. Coronae - Huge,roughly circular volcanic structures. Apparently caused by upwelling mantle material.

12. How does the brightness of Venus compare to other celestial objects? Venus is the third brightest object in the sky. It is 10 times brighter than the brightest star (Sirius).