Other“planets”

Dimensions of the Solar System

1 Astronomical Unit = 1 AU =distance between the Sun and Earth

= ~150 million km or 93 million miles

Masses of objects in the Solar System

The bulk density of a planet tells us about the make-up of its unseen interior (density = weight of 1 cm3; water = 1; rock = 3; iron = 8)

The time it takes for a planet to rotate around its axis of spin determines the length of the day

The sidereal period is the time it takes for a planet to orbit the Sun. It determines the length of the year

The escape velocity is the minimum speed required for an object to overcome the gravitational attraction of a planet and escape into space

The angle between the plane of a planets orbit and the plane of Earth’s orbit around the Sun (called the plane of

ecliptic) is called inclination.

The inclination of the orbits of most planets (except Pluto) is small, i.e., most planets orbit the Sun nearly in the same plane as does Earth (i.e., in the

plane of the ecliptic).

Obliquity is defined as the inclination of the equator of

the planet to its orbital plane

The obliquity of Earth of ~ 23.5° is responsible for the

seasons on the planet.

Planet Obliquity

Mercury 0

Venus 177.4

Earth 23.5

Mars 25.2

Jupiter 3.1

Saturn 26.7

Uranus 98

Neptune 29

Eccentricity (e) is the deviation of a planet’s orbit from a perfect circle (i.e., the orbit becomes more elliptical)

Eccentricity of planet’s orbit around the Sun

Johannes Kepler’s first law of planetary motion: The orbits of the planets around the Sun are ellipses, with the Sun in one focal point.

The orbital eccentricities of the planets around the

Sun are small, except for those of Mercury

and Pluto (i.e., the orbits of most planets are close to a circle)

Johannes Kepler (1571-1630)

Precession of planet’s orbital axis

Changes in obliquity

Changes in eccentricity

Changes in eccentricity and

obliquity in a planet’s orbit can

have profound effects on the climate on the planet, as this

comparison of Earth and Mars indicates

Present and Past Climates of Mars may be very different

Present – cold/dry Past – cold and moist

Albedo is the ability of a planet to reflect

sunlight.

0 albedo means the planet reflects none of the photons reaching it from the Sun; all are

absorbed.

1.0 albedo means all photons are reflected,

and none are absorbed.

Planets with relatively high albedos such as Venus, Earth and the

Gas Giants have highly reflective cloud covers.

To understand what makes our Sun shine for billions of years, you must understand what an element and what an isotope is.

The structure of the oxygen atom. This element has 3 isotopes with masses 16, 17 and 18.

Kepler’s First and Second Laws

1. Orbit of planet is an ellipse with Sun at one focus (other empty)2. Line joining planet to Sun sweeps out equal areas in equal times

Johannes Kepler1571 - 1630

Kepler’s Third Law

3. Square of orbital period increases with the cube of planetary distance