The frequency of the electric powerlines in the U.S. There are third minites but they are seldom used, interestingly enough a third minute is 1/60 of a second. We divide an arcminute into 60 arc seconds, and divide one minute of time into 60 second-minutes, which we have shortened to seconds. Our clock faces are divided into 12 equal parts.Īnd we want to divide a degree of arc into smaller pieces we divide it into 60 arcminutes. When they wanted to divide one of the 360 degrees of a circle into smaller, or minute, parts they broke it into 60 parts called arc minutes.Ĭonsider day and night, the Egyptians divided days into 12 parts and nights into 12 parts, giving us the modern 24 jours per day. The 360 degree circle was passed down to the Greeks and to us. They chose 360 degrees for the circle, which is an approximation to the motion of the sun through the stars each day. and they loved 360 figure its dividers out yourself. They seemed to hate fractions and they loved12, because it can be divided by 2,3,4,6, without any fractional remainders, they also loved 60 because it can be evenly divided by 2,3,4,6,10,12. The Sumerians and Babylonians used a sexigesimal number system based on the number 60. The Ancient Babylonians knew that the sun moved around in the star field about 1/365 of a circle each day. Switching things around if you look at the sun against the background of stars (OK to do this you have to wait for a total solar eclipse which will happen every 300 years if you just stand and wait.) You will see the sun appear to move around you in my model by 1/4 of the circle each day. Which is why the solar day is longer than the sidereal day. which takes about 1/365 of 24 hours which happens to be 4 minutes. So in one day the earth has to rotate 1 + 1/365 rotation. The solar day is 1 + 1/4 rotation which takes 125% of a sidereal day. So the length of the solar day is greater than the length of the sidereal day. Now measure the angle the earth person rotates through when their arm points to the sun the first time. In this model solar system how many days are there in a year? Complete one full rotation while revolving 1/4 of the way around the circle. Start with one arm pointing toward the sun walk around counterclockwise while rotating counterclockwise. Now combine the motions walk in a circle around the central person, the sun, and rotate at the same time. This is called a revolution, not a rotation, and represents one year. Next have the person representing the earth walk in a circle around a stationary person. They rotate through a complete circle of 360 degrees. Have a person point an arm out directly in front of them and rotate around one full time, starting with the arm pointing at a nearby person and ending with the arm pointing at the same person. You have to time the motion of the sun across a north south line.If you do that using shadows or a solar viewer and a north-south power line you will find that it takes 24 hours (plus or minus 20 seconds depending on the time of the year) So why do we say the length of the day is 24 hours? You can measure the rotation of the earth by looking at a distant star, and using a watch to time how long it takes the star to circle the sky and return to the same place I usually observe a star as it passes behind a north-south power line. So when the bottom of the sun touches the horizon as it sets near the equator it will vanish in 2 minutes. (The sun is about the same angular size as a pea held at arms length.)ġ/2 degree/15 degrees/hr = 1/30 hr = 2 minutes. (The angle subtended by the sun is the angle from one edge of the sun to the opposite edge) The sun subtends is 1/2 degree how long does it take for the sun to move through an angle equal to its angular diameter? Using approximate measurememts, the earth's equatorial circumference is 24,000 miles and the earth rotates once in 24 hours so the speed of rotation at the equator isīut we can also measure the angular speed of rotation. How fast does the earth rotate? Linear speed versus angular speed. Both views are used by physicists, but often elementary school teachers label the view that the sun rises or sets as wrong. Viewed from the star polaris, the reference plane attached to the earth revolves around the axis of rotation through the earth, this causes to sun to rise and set on this plane. Viewed from a reference plane tangent to the surface of the earth, the sun rises and the sun sets.
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