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The Planets

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GENERAL INFORMATION ON MERCURY

Mercury is the planet closest to the Sun in our Solar System. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 58.65 Earth days or almost an entire Mercurian year, which is 88 days long), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer.
Mercury is so close to the Sun that you can only see it near sunrise or sunset.

Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core).
SIZE
Mercury is about 3,031 miles (4,878 km) in diameter. It is the second-smallest planet in our Solar System (after tiny Pluto). Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earth's moon.
MASS AND GRAVITY
Mercury's mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth.
The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person on Mercury would weigh 38 pounds. To calculate your weight on Mercury, just multiply your weight by 0.38 (or go the planetary weight calculator)

ATMOSPHERE
Mercury's thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1 x 10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.
Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no "greenhouse effect" on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.
MERCURY'S ORBIT AND DISTANCE FROM THE SUN
Mercury is closest planet to our Sun and the fastest moving planet in our Solar System.

Mercury is just over a third as far from the sun as the Earth is; it is 0.387 A.U. from the sun (on average). Mercury's orbit is very eccentric; at aphelion (the point in the orbit farthest from the sun) Mercury is 70 million km from the sun, at perihelion Mercury is 46 million km from the sun.

There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet's orbit. Since Mercury's axis is directly perpendicular to its motion (not tilted), it has no seasons.

If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth!

TEMPERATURE RANGE
Mercury has a huge range in temperatures. Its surface ranges in temperature from -270°F to 800°F (-168°C to 427°C). During the very long daytime (88 Earth-days long), the temperatures are very high (the second-highest in the Solar System - only Venus is hotter); during the long night, the thin atmosphere lets the heat dissipate, and the temperature drops quickly.
MOONS
Mercury has no moons.
MERCURY-EARTH COMPARISONS

SPACECRAFT VISITS
Mercury was visited by NASA's Mariner 10 in 1973 and 1974. Under half of Mercury's surface was mapped by this spacecraft.
MERCURY'S NAME AND SYMBOL

Mercury was named after Mercury, the mythical Roman winged messenger and escort of dead souls to the underworld. It was named for the speedy Mercury because it is the fastest-moving planet.

INTRODUCTION TO VENUS

GENERAL INFORMATION
Venus is the second planet from the sun in our solar system. It is the hottest planet in our Solar System. This planet is covered with fast-moving sulphuric acid clouds which trap heat from the Sun. Its thick atmosphere is mostly carbon dioxide. Venus has an iron core but only a very weak magnetic field.

This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure.

Venus is also known as the "morning star" or the "evening star" since it is visible and quite bright at either dawn or dusk. It is only visible at dawn or dusk since it is closer to the sun than we are.

Like the moon, Venus' appearance from Earth changes as it orbits around the Sun. It goes from full to gibbous to crescent to new and back.
SIZE
Venus is about 7,521 miles (12,104 km) in diameter. This is about 95% of the diameter of the Earth. Venus is the closest to Earth in size and mass of any of the other planets.
MASS AND GRAVITY
Venus' mass is about 4.87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100-pound person would weigh 91 pounds on Venus.

The density of Venus is 5,240 kg/m3, slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).

LENGTH OF A DAY AND YEAR ON VENUS
Venus rotates VERY slowly. Each day on Venus takes 243 Earth days. A year on Venus takes 224.7 Earth days. It takes 224.7 Earth days for Venus to orbit the sun once. The same side of Venus always faces Earth when the Earth and Venus are closest together.
VENUS' ORBIT AND DISTANCE FROM THE SUN
Venus is 67,230,000 miles (108,200,000 km) from the sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67,230,000 miles = 108,200,000 km from the sun.
Venus rotates in the opposite direction of the Earth (and the other planets, except possibly Uranus). Looking from the north, Venus rotates clockwise, while the other planets rotate counterclockwise. From Venus, the Sun would seem to rise in the west and set in the east (the opposite of Earth). No one knows why Venus has this unusual rotation.

TEMPERATURE ON VENUS
Venus is the hottest planet in our Solar System. Its cloud cover traps the heat of the sun (the greenhouse effect), giving Venus temperatures up to 480°C. The mean temperature on Venus is 726 K (452°C = 870°F).
MOONS
Venus has no moons.
VENUS-EARTH COMPARISONS

FIRST SPACECRAFT TO REACH VENUS
Venera 3 (from the U.S.S.R.) was the first manmade object to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1, 1966 , the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.
VENUS' NAME AND SYMBOL

Venus was named after the Roman goddess of love.

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.

THE 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

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

The 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).

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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.
For more information on the greenhouse effect, click here.
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.

MARS

"The Red Planet"
Mars, the red planet, is the fourth planet from the sun and the most Earth-like planet in our solar system. It is about half the size of Earth and has a dry, rocky surface and a very thin atmosphere.
MARS' SURFACE

The surface of Mars is dry, rocky, and mostly covered with iron-rich dust. There are low-lying plains in the northern hemisphere, but the southern hemisphere is dotted with impact craters. The ground is frozen; this permafrost extends for several kilometers.