1. 1 Lecture 29 Measuring Time on Earth Ozgur Unal

2. 2  What time is it in your hometown at the moment?  What time is it in NYC?

3. 3  It takes 24 hours from when the Sun is highest in the sky until it is highest in the sky again.  Earth spins approximately 360 degrees in 24 hours.  It spins 15 degrees per hour.  A time zone is an area 15 o wide in which the time is the same.  Time zones are modified to fit around city, state, country borders and other key sites.  Check out Figure 5.

4. 4  International Date Line separates two days. If it is Monday to the east of the date line, then it is the same hour on Tuesday to the west of the date line.

5. 5  Earth revolves around the Sun.  It also rotates about its own axis. Rotation:  Rotation is the spinning of Earth on its axis.  The apparent movement of the Sun from noon one day until noon the next day is called a solar day, which is 24 hours.  Solar day is a bit longer than the time it takes Earth to rotate on its axis.  If you measure the time based on when a certain star rises above the horizon until it rises again, you will see a slightly shorter time period: 23 hours 56 minutes 4 seconds.  This time is called sidereal day and is the true measure of the time it takes for Earth to rotate once on its axis.

6. 6 Revolution:  Revolution is the motion of Earth in its orbit around the Sun.  As Earth revolves in its orbit, the Sun appears to move through the skies compared to the seemingly fixed position of the stars.  Sidereal year is the time it takes for the Sun to make one complete trip through the sky in reference to the background stars.  Sidereal year is the same as the solar year.

7. 7 Revolution:  The apparent path of the Sun during a year is called the ecliptic.  Ecliptic is also defined as the plane of Earth’s orbit around the Sun.

8. 8 Lecture 30 Seasons Ozgur Unal

9. 9  Why do we have season on Earth?  The seasonal changes are caused by Earth’s rotation, its revolution and the tilt of the axis.

10. 10  Due to the tilt of the axis of Earth, as it rotates and revolves around the Sun, the angle at which sunlight strikes a particular area changes.  In summers (winters), northern hemisphere gets the sunlight at a higher (lower) angle compared to the southern hemisphere.  Therefore, the northern hemisphere is heated more (less) in summers (winters).

11. 11  Check out the diagram below.  What can you say about the length of daylight and night time at a particular point on Earth?  More (less) hours of daylight in summer (winters) means more (less) heating of the surface and the atmosphere.  This contributes to the high (low) temperatures in summers (winters).

12. 12  Because of the tilt of Earth’s axis, the Sun’s position relative to Earth’s equator constantly changes. Equinox:  Twice a year the Sun is directly over the equator reaching equinox.  During equinoxes, daylight hours equals nighttime hours.  September 22 or 23 and March 20 or 21.

13. 13 Solstice:  The solstice is the point at which the Sun reaches its greatest distance north or south of the equator.  June 21 or 22  Summer solstice  December 21 or 22  Winter solstice

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