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2. Our Place in the Universe
Size and Shape of the Earth
The Earth is spherical (round). How do we know this? How big is the
Earth? There are 24 time zones around the Earth, and each time zone is about 1000 miles (at the
equator), so the circumference is about 24,000 miles. Actually, circumference is 25,000 miles (40,000
km), diameter 8000 miles (13,000 km)
Size of the Moon
The Moon is about one fourth (27%) the diameter of the Earth. About 50 Moons
would fit inside the Earth. If the Earth is a basketball, the Moon is a tennis ball.
Distance to the Moon
The Moon is about thirty Earth-diameters away from the Earth. A commercial
Boeing 747 would take eighteen days to get to the Moon. The Apollo missions took about three days to
fly to the Moon. Light (or radio waves) takes 1.3 second to travel from Moon to Earth.
Definition of a planet
A planet is a large, spherical natural satellite of a star that “dominates” its orbit—
has “cleared” its orbit. The planets in our solar system are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune. All the planets in our Solar System orbit in the same plane (like on the surface of
a pizza).
Definition of “moon”
A moon is a substantial natural object (satellite) in orbit around a planet. All of
our planets have at least one moon, except Mercury and Venus.
Distance to the Sun
93 million miles (150 million km). A 747 would take 20 years to fly to the Sun.
Light takes 8.3 minutes to come from the Sun. If the Sun is the size of a basketball, the Earth is half
the size of a BB, located 28 yards away. The distance from Earth to Sun is defined as the Astronomical
Unit.
Definition of Star
Our Sun is the star in the middle of our Solar System. A star is a giant ball of
mostly hydrogen and helium that produces energy in its core by nuclear fusion.
The Astronomical Unit
The Astronomical Unit is defined as the average distance from Sun to Earth
(93 million miles, 150 million kilometers). It’s a useful unit when describing distances in a solar system.
The Earth is 1 AU (one Astronomical Unit) from the Sun. Jupiter is 5.2 AU from the Sun. Pluto is 40
AU from the Sun.
Distance to the next closest star (after the Sun)
The nearest star is called Proxima Centauri (it’s in a
star system of three stars, called Alpha Centauri). Proxima Centauri is 39,900,000,000,000 km away
(28,500,000,000,000 miles) = 3.99 x 1013 km. This method of writing numbers is called scientific
notation. This is 4.22 light years (4.22 ly). A light year is the distance that light travels in one year.
(equaling 9.46 x 1012 km). Book analogy: If the Sun is a pinhead, the next star is another pinhead 35
miles away. This shows that the universe is made mostly of empty space.
Definition of a solar system
A solar system contains one (or a few) stars with all the things in orbit
around it: planets, dwarf planets, moons, asteroids, Kuiper belt objects, comets, meteoroids, etc. Our
solar system is called the Solar System
How we measure the distance to nearby stars
The geometric principle of parallax allows us to
measure the distance to the nearby stars.
Stars move around the galaxy
This illustration shows how Bernard’s Star has been moving for the
past 20 years. Most stars are too far away for their motion to be noticeable.
Definition of a galaxy
A galaxy is an isolated grouping of billions of stars, held together by its own
gravity
Map of the Milky Way
The Local Group
100,000 light years from edge to edge
Our Galaxy is a member of a group of galaxies, called the Local Group. The
closest “big” galaxy to us is the Andromeda Galaxy, 2.65 million light years away.
Super Cluster
Even the clusters of galaxies are lumped together in associations, called super
clusters. Our super cluster is called the Local Supercluster.
The Universe
Everything that exists is in the Universe. The Universe has an infinite number of
galaxies, and an infinite number of stars and planets. It has no limit, no edge, no boundaries. But it
does have an age—13.7 billion years.
Our Cosmic Address
Planet Earth, The Solar System, The Milky Way Galaxy, The Local Group of
Galaxies, The Local Supercluster, The Universe
Questions to be Answered:
•
What is the shape, and how big is the Earth?
•
How big is the Moon compared to the Earth?
•
What’s the Moon’s orbit? (and what is a moon…?)
•
How far away is the Sun? (and what’s a star…?)
•
What is an Astronomical Unit, and how do we use it?
•
What are the positions and orbits of the other planets? (and what’s a planet…?)
•
What else is in our Solar System?
•
How far is the next closest star? (how do we know that…?)
•
What is a light year, and how do we use it?
•
Where are we in our Galaxy? (and what’s a galaxy…?)
•
What is our cosmic address?