# solar system

### Solar System

Before the true distance of the Earth from the Sun was known,
and what the true distances in the Solar System were,
relative distances were measured by the Astronomical Unit.
It still is the most meaningful way to understand the scale of the Solar System

The Astronomical Unit: The mean distance of the Earth from the Sun.  This distance is defined to be 1 A.U.

Bode's Rule

In 1772, the German astronomer Johann Bode found an empirical rule, dubbed Bode's Rule, that does a remarkable job of "predicting" planetary positions in the Solar System.

1. Write down a sequence of 4s, one for each planet except Neptune, including one for the asteroids.
2. Add the sequence 0, 3, 6, 12, 24, ….
3. Divide these numbers by 10.

The result is the relative distance of each object from the Sun, to a surprising degree of accuracy!

What Is A Planet?

We all have heard that the Sun has nine planets circling around it, but what exactly is a planet?

e.g., Is Pluto a planet?

In the 1990s, many "asteroids" or "cometary nuclei" approaching 1/4 Pluto's size were
discovered.
Pluto crosses Neptune's orbit, like an asteroid or a comet.
Pluto is only 1/2 the size of our own Moon.  Seven planetary satellites are larger than Pluto.
Pluto has a highly inclined orbit.

Planet: A non-stellar body larger than a certain size (roughly 1,000 to 3,000 km), orbiting around a star as its primary host.

A List Of Important Numbers/Constants:

Gravitational Constant: G = 6.67 × 10-11 N·m2/kg2
Astronomical Unit:   1 AU = 1.49 × 1011 m
Velocity Of Light:  c = 3.00 × 106 m/s
Mass Of Sun:    M¤ = 2.00 × 1030 kg
Radius Of Sun:   R¤ = 6.96 × 108 m
Mass of Earth:   MÅ = 5.98 × 1024 kg
Radius Of Earth:   RÅ = 6.38 × 106 m

A Survey Of The Planets: Mercury

Notes:  Very Small Magnetic
Field, Past Cratering,
Inter-Crater Plains - Past
Geologic Activity.

A Survey Of The Planets: Venus

Notes: Thick Atmosphere,
Little/No Magnetic Field,
Evidence of Past/Present
Geological Activity.

A Survey Of The Planets: Earth

Notes: Liquid Water In
Abundance, Life!

Earth's Moon

Moon Data:
Diameter
3475.6 Km  (2159.7 miles)
Rotational period
27.322 days
Sidereal revolution period
27.322 days
Sinodic revolution period
29d12h44m2,9s
Mass (Earth=1)
0.0123
Volume (Earth=1)
0.0203
Gravity (Earth=1)
0.1653
Mean distance from Earth
384,400 Km  (238,862 miles)
Minimum distance from
Earth
356,410 Km  (221,469 miles)
Maximum distance from
406,697 Km  (252,717 miles)

A Survey Of The Planets: Mars

Notes: Evidence Of Abundant
Water In The Past,
Spectacular Geological
Features.

A Survey Of The Planets: Jupiter

Notes: Largest Planet, "Mini
Solar System", Fast Rotation.

A Survey Of The Planets: Saturn

Notes: Very Low Density.

A Survey Of The Planets: Uranus

Notes: Rotates On Its Side!

A Survey Of The Planets: Neptune

Notes: Fantastic Blue
Color!

A Survey Of The Planets: Pluto

Notes: "Oddball" Planet -
Small Size For Location, Highly
Inclined Orbit.

The Solar System: Minor Bodies

There is a lot of debris in the solar system, including meteoroids, asteroids and comets.
Asteroids:  A small planetary body composed mostly of rock or metals; most orbit in the asteroid belt between Mars and Jupiter.  All known asteroids have diameters less than 1,000 km.  Lower size limit often taken to be about 10 m.  Many unaltered since their formation - give clues to Solar System formation.

Meteoroids:  A small, extraterrestrial, solid body.  Term usually used for bodies smaller than a
kilometer, and frequently millimeters, in size.  Objects destined to become meteors.

Comets:  An ice-rich planetesimal that can emit an observable gaseous halo when its ice is warmed by the Sun.  Most comets spend their time in the outer Solar System, making it difficult to distinguish them from normal asteroids.

Comparison of Planet Properties