The document provides an overview of our planetary neighborhood by defining key terms like astronomical unit and discussing objects in our solar system like the Earth, Moon, planets, and Sun. It describes the size, composition, and orbits of these bodies, as well as distances measured in astronomical units. Diagrams and images are included to illustrate relative sizes and positions within the solar system.

1. The Astronomy Picture of the Day

2. Our Planetary Neighborhood

3. Outline
• Astronomy
• The Earth
• The Moon
• The Planets
• The Sun
• The Solar System
• The Astronomical Unit

4. Astronomy (from Wikipedia)
Astronomy (Greek: αστρονομία = άστρον +
νόμος, astronomia = astron + nomos, literally,
"law of the stars") is the science of celestial objects
and phenomena that originate outside the Earth's
atmosphere, such as stars, planets, comets,
auroras, galaxies, and the cosmic background
radiation. It is concerned with the formation and
development of the universe, the evolution and
physical and chemical properties of celestial objects
and the calculation of their motions.

5. The Earth - symbol ♁
• Our home planet.
• Spinning ball of rock &
metal, coated with a thin
layer of gas & liquid.
• Huge by human
standards, but it is one
of the smaller bodies in
the cosmic landscape.
orbit: 149,600,000 km (1.00 AU) from Sun
diameter (D♁): 12,756.3 km
mass (M♁): 5.972×1024 kg

6. The Moon -
symbol ☾
orbit: 384,400 km from Earth
diameter (D☾): 3476 km
mass (M☾): 7.35×1022 kg
• Our nearest neighbor, but very different from the
Earth
• Our only natural satellite
• It marks the present limit of direct human exploration
of space
• Its appearance is different if we are either using our
naked eyes (smooth), or a telescope (rocky)

7. The Planets
• In order of increasing distance from
the Sun, the eight planets are:
Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, & Neptune.
These worlds have dramatically
different sizes and landscapes.
• They range in size from Jupiter
(RJupiter = 11 R♁), down to
Mercury(RMercury = 0.4 R♁).

8. The Planets cont.
• Our two nearest neighboring planets
are Venus (RVenus = 0.95 R♁), & Mars
(RMars = 0.53 R♁). Despite their
similarities in size & in distance from
the Sun, Venus & Mars have
dramatically different atmospheres.
• On Venus we would be crushed &
cooked by its intensely hot, dense
atmosphere, whereas on Mars we would
suffocate & freeze.

9. Picture of the eight planets & Pluto (‘dwarf’ planet)
along with silhouettes showing their correct relative size

10. The Sun - symbol
• It is a star.
• Much bigger than the planets.
• It generates energy in its core by converting
hydrogen into helium (nuclear fusion) at a
furious rate.
diameter (D ): 1,390,000 km.
mass (M ): 1.989×1030 kg
temperature: 5800 K (surface), 15,600,000 K (core)

11. The Sun cont.
• The Sun’s energy is enormous, but limited:
it has already lasted more than 4 billion
years, and evidence indicates that the Sun
will run out of fuel eventually, in about
another 5 or 6 billion years.
diameter (D ): 1,390,000 km.
mass (M ): 1.989×1030 kg
temperature: 5800 K (surface), 15,600,000 K (core)

12. The Solar System
• The Sun & the bodies
orbiting it form the
Solar System, bound
together by the enormous
gravity of the Sun.
• In addition of the eight
planets, the Solar
System is filled with a
vast number of smaller
bodies: satellites
(moons) orbiting the
planets and asteroids &
comets orbiting the Sun.

13. Sketch of the orbits of the planets & Pluto in
our Solar System in 2006

14. A scale model of the Solar System with the Sun the
size of a grapefruit

15. The Astronomical Unit
• Distances within the Solar System are not
generally given in miles or kilometers. They
are given in terms of the astronomical unit
(AU), which is defined as the average
distance Earth-Sun:
• 1 AU = 92,955,800 miles ~ 93,000,000 miles
= 149,000,000 km
- Distance Jupiter-Sun = 5.2 AU
- Distance Neptune-Sun = 30 AU
- Distance α Centauri-Sun = 270,000 AU

16. View of the Solar System based on a series of real
images made by the Voyager I spacecraft.
A sketch of the orbits of the planets

17. Key Terms
• astronomical unit (AU):
• metric system:
• planet:
• satellite:
• Solar System:
• star:
• unit: