Moon

MOON...what do you know about moon? why the moon is vital for life?
let's explore about moon !
first... we should know the characteristics of moon.

The Moon is an astronomical body that orbits planet Earth, being Earth's only permanent natural satellite.

Distance to Earth: 384,400 km

Radius: 1,737 km

Gravity: 1.622 m/s²

Orbital period: 27 days

Polar radius: 1736.0 km (0.273 of Earth's)

Axial tilt: 1.5424° to ecliptic; 6.687° to orbit plane;

Miguel Claro recently sent Space.com this beautiful image of the moon and Earthshine taken from Almada, Portugal on Feb. 1, 2014. “I could see the night side of the moon very well illuminated by the Earth reflected light, like if it was full moon,” Claro wrote Space.com in an email. “This impressive phenomenon known as Earthshine, was described and drawn for the first time, by the great Leonardo Da Vinci about 500 years ago in his book Codex Leicester.”

Credit: Miguel Claro | www.miguelclaro.com

HOW BIG IS THE MOON? 

Earth's moon is the brightest object in our night sky. It appears quite large, but that is only because it is the closest celestial body. The moon is a bit more than one-fourth (27 percent) the size of Earth. Did you know that Earth's moon is the fifth largest moon in the solar system.

The moon's mean radius is 1,079.6 miles (1,737.5 kilometers). Double those figures to get its diameter: 2,159.2 miles (3,475 km). The moon's equatorial circumference is 6,783.5 miles (10,917 km).

The moon's surface area is about 14.6 million square miles (38 million square kilometers), which is less than the total surface area of the continent of Asia (17.2 million sq mi or 44.5 million sq km).

Mass, density and gravity

The moon's mass is 7.35 x 1022 kg, about 1.2 percent of Earth's mass. Put another way, Earth weighs 81 times more than the moon. The moon's density is 3.34 grams per cubic centimeter (3.34 g/cm3). That is about 60 percent of Earth's density. The moon is the second densest moon in the solar system; Saturn's moon Io is denser, with 3.53 g/cm3.

The moon's gravitational force is only about 17 percent of Earth's gravity. A 100-pound (45 kg) person would weigh only 17 pounds (7.6 kg) on the moon. A person who can jump up 10 feet on Earth would be able to jump almost 60 feet on the moon.

The Milky Way

Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dustbound together by gravity. They come in a variety of shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy. Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars. These stars form a large disk whose diameter is about 100,000 light years. Our Solar System is about 25,000 light years away from the center of our galaxy – we live in the suburbs of our galaxy. Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes 250 million years for our Sun and the solar system to go all the way around the center of the Milky Way.

The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky (and, as mentioned above, is how the Milky Way got its name). This band of stars can be seen with the naked eye in places with dark night skies. That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat.

Several different telescopes, both on the ground and in space, have taken images of the disk of the Milky Way by taking a series of pictures in different directions – a bit like taking a panoramic picture with your camera or phone. The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band.

Another clue comes when astronomers map young, bright stars and clouds of ionized hydrogen in the Milky Way's disk. These clouds, called HII regions, are ionized by young, hot stars and are basically free protons and electrons. These are both important marker of spiral arms in other spiral galaxies we see, so mapping them in our own galaxy can give a clue about the spiral nature of the Milky Way. There are bright enough that we can see them through the disk of our galaxy, except where the region at the center of our galaxy gets in the way.

There has been some debate over the years as to whether the Milky Way has two spiral arms or four. The latest data shows that it has two arms, as shown in the artist's illustration below.

The Milky Way is the galaxy that contains our Solar System. The descriptive "milky" is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars

Number of stars: 100–400 billion (2.5 × 10¹¹ ± 1.5 × 10¹¹)

Diameter: 100–180 kly (31–55 kpc)

Constellation: Sagittarius

Oldest known star: ≥13.7 Gyr

Sun's Galactic rotation period: 240 Myr

Sun's distance to Galactic Center: 26.4 ± 1.0 kly (8.09 ± 0.31 kpc