The Earth is a sphere that rotates on its axis that passes through the North and South poles. The time it takes for the Earth to make a complete rotation is approximately 24 hours (exactly 23.934 hours). This rotation results in daytime when an area is facing the Sun and nighttime when an area is facing away from the Sun.

ROTATION

Mean sidereal day: the period during which

Earth makes a complete rotation on its axis

(absolute rotation). The time interval of the

mean sidereal day is 86164.0905382 seconds.

Mean solar day: The period of time

between two successive transits of the mean

sun; the standard for the 24-hour day

measured from midnight to midnight

HOW FAST IS THE EARTH SPINNING AROUND ITS AXIS?

At the equator, the Earth's surface moves 40,000kilometers in 24 hours. That is a speed of about1040 miles/hr (1670 km/hr or 0.5 km/sec).

This is calculated by dividing the circumference ofthe Earth at the equator (about 24,900 miles or40,070 km) by the number of hours in a day (24).

As you move toward either pole, this speeddecreases to almost zero (since the circumferenceof the spinning circle at the extreme latitudesapproaches zero).

The Earth revolves around the Sun once every 365.25 days. Its velocity in space in its orbit is about 18.5 miles per second. The shape of the orbit is an ellipse

REVOLUTION

A tropical year (also known as a solar year) is thelength of time that the Sun takes to return to thesame position in the cycle of seasons, as seenfrom Earth; for example, the time from vernalequinox to vernal equinox, or from summersolstice to summer solstice.

The sidereal year is the time taken for the Sun toreturn to the same position with respect to thestars of the celestial sphere. It is the orbital periodof Earth, equal to 365.25636042 The sidereal yearis 20 minutes and 24 seconds longer than thetropical year.

HOW FAST IS THE EARTH REVOLVING AROUND THE SUN?

The Earth revolves around the Sun at a speed of about 18.5 miles/sec (30 km/sec).

The Earth orbits, on average, 93 million miles (149,600,000 km) from the Sun (this distance is defined as one Astronomical Unit (AU)), taking one year to complete an orbit.

PRECESSION

One consequence of precession isthat the north star Polaris isdrifting. It is only "north star" bycoincidence today. Vega will beour north star for a time in thedistant future.

Another consequence is that precession introduces a difference between a sidereal (real) year and a tropical (by the sun) year because during the course of one year the position of the equinox changes slightly.

"precession" was discovered by Greek astronomer Eratosthenes when he compared his own circa 200 BC records with older charts

HOW FAST IS OUR SOLAR SYSTEM MOVING WITHIN THE MILKY WAY GALAXY?

It takes the sun (and our solar system) roughly 200-250million years to orbit once around the Milky Way. Inthis orbit, we (and the rest of the Solar System) aretraveling at a velocity of about 155 miles/sec (250km/sec).

The sun is about 26,000 light-years from the center ofthe Milky Way Galaxy, which is about 80,000 to 120,000light-years across (and less than 7,000 light-years thick).We are located on one of its spiral arms, out towardsthe edge

HOW FAST IS THE MILKY WAY GALAXY MOVING WITHIN THE LOCAL GROUP OF GALAXIES?

The Milky way Galaxy is just one galaxy in a group of galaxies called the Local Group. Within the Local Group, the Milky Way Galaxy is moving about 185 miles/sec (300 km/sec).

Grids an parallels