THE SKY AND SEASONS

How We Locate Objects on the Sky

Constellations used to be broad brush areas of the sky where ancient people's saw pictures. Modern astronomer break up the sky more formally. From east to west, we measure angles in the sky as Right Ascension. This is like longitude on the Earth. From north to south, we measure angles as Declination. This is like latitude on Earth. Like every city has a latitude and longitude on Earth, every object in the sky has Right Ascension and Declination in the sky.

Constellations and Asterisms

Constellations are defined by fixed regions of Right Ascension and Declination, just like the boarders of counties and states are defined by regions of latitude and longitude. Like states on a map, the constellations can have complex boundaries.

Asterisms are highly popular and recognizable parts of constellation. For example, the Big Dipper asterism is part of the constellation Ursa Major (The Big Bear). The Great Square asterism is part of the constellation Pegasus (The Flying Horse).

Star Names

Star names are part Greek and part Latin. The Greek part refers to its relative brightness within the constellation. (Alpha = 1st brightest, Beta = 2nd brightest, ...etc.)

If they are in the constellation Orion, then the brightest star is Alpha Orioinis. The Latin possessive suffix is employed. Another example is the second brightest star in the constellation of Canis Major (The Big Dog), which goes by the name Beta Canis Majoris.

Orientation of Earth and Apparent Motions of Star

The Earth is tilted 23.5 degrees relative to the plane of its orbit. In fact, this tilt is fixed in space. The axis through the Earth toward the north points directly at a star called Polaris, known as "The North Star". This is a chance circumstance. However, it is used for celestial navigation. The angle above your northern horizon to the north star is equal to your latitude on Earth. If you reside at the Earth's equator (lattitude zero), then the North Star will appear precisely on your northern horizon (zero degrees above the horizon). If you reside at the North Pole (lattitude 90 degrees), then the North Star will appear at your zenith (directly over head), which is 90 above your horizon. If you reside below the Earth's equator (negative lattitude), you will not be able to see the North Star because it will be below your northern horizon. So, in the southern hemisphere you cannot use the North Star to estimate your lattitude on Earth, but its absence in the sky tells you that you are below the equator.

The motions of stars are because you live on a rotating platform, Earth. As Earth rotates, stars rise in the east and set in the west. The angle that they rise at depends upon your latitude on Earth. If you are in the northern hemisphere, you see some stars that never rise or set. These are called circumpolar stars.

Seasons on Earth

Seasons on the Earth are due to the fact that the Earth is tilted relative to the plane of its orbit. When the north axis is tilted toward the sun, it is summer in the northern hemisphere and winter in the southern hemisphere. When the south axis is tilted toward the sun, it is summer in the southern hemisphere and winter in the northern hemisphere. Why? The season depends upon the angle at which the sunlight strikes the Earth. The more direct the sunlight, the more energy the light deposits per unit area (say, per square meter) onto Earth (this is summer); the more angled the incoming sunlight, the less energy the light deposits per unit area on the Earth (this is winter).