2. Earth is the third planet from the Sun, after Mercury and
Venus. The average distance between Earth and the Sun is
150 million km (93 million mi). Earth and all the other
planets in the solar system revolve, or orbit, around the
Sun due to the force of gravitation. The Earth travels at a
velocity of about 107,000 km/h (about 67,000 mph) as it
orbits the Sun. All but one of the planets orbit the Sun in
the same plane—that is, if an imaginary line were
extended from the center of the Sun to the outer regions
of the solar system, the orbital paths of the planets would
intersect that line. The exception is the dwarf planet
Pluto, which has an eccentric (unusual) orbit. ®
INTRODUCTION
NASA
3. The Early Earth
Life originated on Earth about four billion years
ago, when oceans dotted with volcanic islands
covered most of Earth’s surface and continents
were very small. The air was hot and contained
almost no breathable oxygen. The Moon was much
closer to Earth, and a day was less than 15 hours
long. Meteorites fell more frequently, and there
was more volcanic activity than there is today.
4. Earth from the Moon
In the late 1960s, people saw for the first time what Earth
looked like from space. This famous photo of Earth was
taken by astronauts on the Apollo 8 mission as they
orbited the Moon in 1968.
5. Equatorial radius 6,378 km
Equatorial inclination 23.5°
Mass 5.97×1024 kg
Average density 5.5 g/cm3
Rotational period 0.997 days
Orbital period 1 year
Average distance from
the Sun
149.6 million km
Perihelion 147.1 million km
Aphelion 152.1 million km
Orbital eccentricity 0.0167
Orbital inclination 0.0003°
Moons 1
6. ROTATION OF EARTH
Earth’s orbital path is not quite a perfect circle but
instead is slightly elliptical (oval-shaped). For example,
at maximum distance Earth is about 152 million km
(about 95 million mi) from the Sun; at minimum
distance Earth is about 147 million km (about 91 million
mi) from the Sun. If Earth orbited the Sun in a perfect
circle, it would always be the same distance from the
Sun.
7. REVOLUTION OF EARTH
As Earth revolves around the Sun, it rotates, or spins, on its
axis, an imaginary line that runs between the North and
South poles. The period of one complete rotation is defined
as a day and takes 23 hr 56 min 4.1 sec. The period of one
revolution around the Sun is defined as a year, or 365.2422
solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves
along with the Milky Way Galaxy as the Galaxy rotates and
moves through space. It takes more than 200 million years
for the stars in the Milky Way to complete one revolution
around the Galaxy’s.
