Finding our way on Earth

We use a coordinate system:  latitude and longitude

Great Circle:  a circle on the surface of a sphere whose center is at the center of the sphere. For example, the equator is a great circle

Equator:  splits the earth into north and south

Meridians:  they pass through the north and south poles; they specify east - west location; called longitude

Longitude is measured in degrees (remember a circle has 360 degrees); zero degrees has been arbitrarily assigned to Greenwich, England

Longitude is measured to east or west of the Greenwich meridian from 0° to 180°

Example:  Schenectady is located at longitude of 73.9° west

Latitude:   describes the number of degrees of arc the location is from the equator either north or south, from 0° to 90°

Example:  Schenectady is located at latitude 42.9° north

Schenectady coordinates: 73.9° w  42.9° n

Isn't it nice to know where you are?

Finding our way around the sky

We use a similar coordinate system to describe points in the sky

Celestial sphere:  early Greeks thought the Earth was enclosed in a huge crystalline sphere to which the stars and other objects seen in the sky were attached. This is NOT the case, but the idea of the celestial sphere provides us with a good working model to describe the sky

Celestial equator:  the projection of the Earth's equator onto the celestial sphere

Right ascension (RA):  the projection of the Earth's lines of longitude onto the celestial sphere

RA is measured in hours (24 of them), minutes and seconds. Each hour is actually 15 degrees of arc (24 x 15 = 360°). Sixty seconds makes a minute, and sixty minutes makes an hour.

Example: The right ascension of the middle star in Orion's belt, Alnilam, is written
                  RA 05h 36m 20s

The point of origin of right ascension is arbitrarily assigned to a point in the sky called the vernal equinox. RA is measure to the east of the vernal equinox from 0h to 23h 59m 59s

The vernal equinox is indeed a day (the first day of spring) but it is also a place in the sky, the point where the ecliptic crosses the celestial equator

Ecliptic:  the apparent path of the Sun through the sky over the course of a year

Declination (Dec):  the projection of the Earth's lines of latitude onto the celestial sphere

Dec is measured in degrees, minutes and seconds. It is measured north or south of the celestial equator from 0° to 90°

Example: The Dec of the middle star in Orion's belt is written
                  Dec -01° 11' 50"
Note:  the minus sign in front of the 01° indicates that it is south of the celestial equator. It would have a plus sign (+) or nothing if north. A minute is indicated by a single quotation mark rather than the letter m as in RA, and a second is idicated by a double quotation mark rather than the letter s as in RA.

Alnilam's celestial coordinates are written:  RA 05h 36m 20s    Dec -01° 11' 50"

Seasons

We have seasons here on Earth because the Earth's axis is tilted relative to the plane of its orbit.

The axis is tilted 23 1/2°, and points the same direction as the Earth orbits around the Sun. Therefore, the axis tilts toward the Sun twice a year, once for the northern hemisphere and six months later for the southern hemisphere.

When the axis is pointing toward the Sun, that hemisphere gets more direct sunlight (summer). The hemisphere that is pointing away from the Sun at that same time receives less direct sunlight (winter).

Twice in the orbit the Earth's axis does not point at the Sun, but is aligned sideways to the Sun. At these times the northern and southern hemispheres of the Earth receive the same amount of sunlight. Those times are called spring and fall.

But that's not all, folks....

The length of time that the Sun is above the horizon also dictates the amount of heating of the Earth.

Winter: 

Days are short! For example, on December 22 (winter solstice) the Sun rises at 7:23 am and sets at 4:25 pm in Schenectady. The Sun is above the horizon for only 9 hours.

Sun rises south of east (azimuth ~ 124° compared to azimuth of 90° at due east) and sets south of west (azimuth ~ 235° compared to azimuth of 270° at due west); it's path through the sky is short, so it is not above the horizon for long. Also, the Sun doesn't get too high in the sky, only about 24° above the horizon.

Page created by C. Gino on 09/11/2003

Page last updated 09/11/2003