INTRODUCTION TO THE EARTH
The Earth is the third planet from the Sun in our Solar System. It
is the planet
we evolved on and the only planet in our Solar System
that is known to support life.
SIZE
The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is
the fifth-largest planet in our Solar System (after Jupiter, Saturn,
Uranus, and
Neptune).
Eratosthenes (276-194 BC) was a Greek scholar who
was the first person to determine the circumference of the Earth. He
compared the midsummer's noon shadow in deep wells in Syene (now
Aswan on the Nile in Egypt) and Alexandria. He properly assumed that
the Sun's rays are virtually parallel (since the Sun is so far
away). Knowing the distance between the two locations, he calculated
the circumference of the Earth to be 250,000 stadia. Exactly how
long a stadia is is unknown, so his accuracy is uncertain, but he
was very close. He also accurately measured the tilt of the Earth's
axis and the distance to the sun and moon.
HE
MOON
The Earth has one
moon.
The diameter of the
moon
is about one quarter of the diameter of the Earth.
The moon may have once been a part of the
Earth; it may have been broken off the Earth during a catastrophic
collision of a huge body with the Earth billions of years ago.
MASS, DENSITY, AND ESCAPE VELOCITY
The
Earth's mass is about 5.98 x 1024 kg.
The Earth has an average density of 5520 kg/m3 (water has
a density of 1027 kg/m3). Earth is the densest planet in
our Solar System.
To escape the Earth's gravitational pull, an object must reach a
velocity of 24,840 miles per hour (11,180 m/sec).
LENGTH OF A DAY AND YEAR ON EARTH
Earth rising over the moon.
|
Each day on Earth takes 23.93 hours (that is, it takes the Earth
23.93 hours to rotate around its axis once - this is a
sidereal
day). Each year on Earth takes 365.26 Earth days
(that is, it takes the Earth 365.26 days to orbit the Sun once).
The Earth's rotation is slowing down very slightly over time, about
one second every 10 years.
THE EARTH'S ORBIT
he Earth orbits, on average, 93 million
miles (149,600,000 km) from the Sun. This distance is defined as one
Astronomical Unit (AU). The Earth is closest to the Sun (this is
called perihelion) around January 2 each year (91.4 million miles =
147.1 million km); it is farthest away from the Sun (this is called
aphelion) around July 2 each year (94.8 million miles = 152.6
million km).
Orbital Eccentricity
The Earth' orbital eccentricity is 0.017; it has an orbit that is
close to being circular.
THE EARTH'S AXIS TILT AND THE SEASONS
The
Earth's axis is tilted from perpendicular to the plane of the
ecliptic by 23.45°. This tilting is what gives us the
four
seasons of the year:
Summer, Spring, Winter and Autumn. Since the axis is tilted,
different parts of the globe are oriented towards the Sun at
different times of the year. This affects the amount of sunlight
each receives.
SPEED
At
the equator, the Earth's surface moves 40,000 kilometers in 24
hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is
calculated by dividing the circumference of the Earth at the equator
(about 24,900 miles or 40,070 km) by the number of hours in a day
(24). As you move toward either pole, this speed decreases to almost
zero (since the circumference at the extreme latitudes approaches
zero).
The Earth revolves around the Sun at a speed of about 30 km/sec.
This compares with the Earth's rotational speed of approximately 0.5
km/sec (at middle latitudes - near the equator).
The size of the atmosphere in this illustration is greatly
exaggerated in order to show the greenhouse effect. The
Earth's atmosphere is about 300 miles (480 km) thick, but
most of the Earth's atmosphere is within 10 miles (16 km) of
the Earth's surface. |
TEMPERATURE
ON EARTH
The temperature on Earth ranges from between -127°F to 136°F
(-88°C to 58°C; 185 K to 311 K). The coldest recorded temperature
was on the continent of Antarctica (Vostok in July, 1983). The
hottest recorded temperature was on the continent of Africa (Libya
in September, 1922).
The greenhouse effect traps heat in our atmosphere. The
atmosphere lets some infrared radiation escape into space; some is
reflected back to the planet.
ATMOSPHERE
The
Earth's atmosphere is a thin layer of gases that surrounds the
Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03%
carbon dioxide, and trace amounts of other gases.
The atmosphere was formed by planetary degassing, a process
in which gases like carbon dioxide, water vapor, sulphur
dioxide and nitrogen were released from the interior of the Earth
from volcanoes and other processes. Life forms on Earth have
modified the composition of the atmosphere since their evolution.
