1. Earth’s Motion
Days and Years

2. What is Time?
The measurable period during which an action, process or condition exists or continues.
the indefinite continued progress of existence and events in the past, present, and future regarded as a whole.
a point of time as measured in hours and minutes past midnight or noon.
"the time is 9:30”

3. What is Time? Basic units of time…
Seconds, Minutes, Hours, Days, Years
Forward Motion
Relativistic time : decreases with increasing velocity with respect to outside observer.

4. How do we measure time?
Presently, time is measured in seconds as corresponding to the frequency [9,192,631,770 hertz (Hz = cycles/second)] of radiation emitted from the 133 Cs atom as excited outer electrons change or “jump” energy states.

5. How do we measure time? Sunrise to sunset?
Has the earth’s rotational speed been constant throughout geologic time?
Solar day is 24 hours (4.5 by ago the Earth day was 6 hours and 620 my ago it was 21.9 hours)
Note: the Earth is slowing down by roughly 1.7 milliseconds per century.

6. Cesium 133 Wrist Watch $

7. Development of Time Zones
What if everyone kept the same time…then 12 midnight for some would be dark and for others it would be light.
What led to the development of time zones?
Synchronization of human activities over long distances led to the standardization of local times.
How were the time zones determined?
It takes the Earth approximately one hour to rotate 15 degrees.
Because of this, time zones are separated by generally 1 hour or 15 degrees longitude.

8. Different parts of the world experience different times of day as the Earth rotates.
TIME ZONES which can be used to calculate the time of day in any given part of the world.

9. Sidereal Period
Sidereal Period: The orbital period of one object about another measured with respect to the background stars.

10. The Sidereal Day
It takes 23 hours, 56 minutes and 4 seconds for the Earth to complete one rotation with respect to the background stars.
Notice that a sidereal day falls short of 24 hours.
As the Earth rotates, it also revolves around the Sun at a rate of nearly 1 degree per day.
To compensate for this motion, the Earth must rotate an additional 4 minutes and 4 seconds to complete a full solar day (from the perspective of the Sun).

11. Mean Solar Day vs. Sidereal Day
Solar Day: One complete rotation with respect to the Sun. The average of this defines our 24-hour day. That is why it is referred to as a mean solar day.
Sidereal Period: The orbital period of one object about another measured with respect to the stars.
Sidereal Day: the length of one rotation of the Earth with respect to the background stars. Our sidereal day is 23 hours 56 minutes 4.1 seconds.

12. Days Why is a mean solar day longer than a sidereal day?

13. The Sidereal Year
The sidereal year is the time taken for the Earth to orbit the Sun and to return to the same position with respect to the background stars.
A sidereal year is equal to mean solar days.

14. The significance between sidereal and mean solar day
The stars rise approximately 4 minutes earlier each night. This is due to the difference in the length of the mean solar day (24 hours) and sidereal day (23 hours 56 minutes 4.1 seconds).
 With each night that passes, the Earth falls four minutes short of returning to its starting point with reference to the Sun. So each night, the Earth falls behind by four minutes.

15. The significance between sidereal and mean solar day
So, from your viewing location at the same time each night, the Earth points towards a different part of the universe, giving us a slightly different view of the stars.
 1st night: you see a constellation at a specific coordinate at a specific time
2nd night: you see the constellation at the same coordinate, but in order to see this, you must view the night sky four minutes earlier.
3rd night: you see the constellation, but eight minutes earlier…and so on…

16. Year Tropical year Sidereal year
One revolution of Earth around the Sun relative to the position of the equinoxes.
mean solar days
Sidereal year
One revolution of Earth around Sun relative to fixed stars
mean solar days
Difference between the tropical and sidereal year is the caused by the precession of the equinoxes (Earth’s gradual precession causes the orientation of the equatorial plane to shift slightly relative to the ecliptic plane.

17. Leap Year
To compensate for the extra .24 days of the tropical year ( mean solar days)—spring to spring or vernal equinox to vernal equinox.
Add a year if the year is divisible by 4.
Skip all centuries unless it is divisible by 400.

18. What is the analemma?
The figure-eight pattern signifying the path of the Sun over the course of a year.
What causes this?
The analemma is a product of the variation in the speed of the Earth’s orbit. (the Earth’s orbit is slightly elliptical)
Demo: Chapter 9 (Analemma)
Click on analemma-A
Note the path of the sun over the course of a year—
If the Earth has a circular orbit, how would this change the pattern of the Sun during the course of a year?

19. Starry Night Demo From Chapter 4 Lab (Earth’s Orbital Motion)
Orbit A: the view as time passes shows no change in the location of the background stars. This represents successive sidereal days, not mean solar days. If you click one step ahead to the next day, you will see that the time is four minutes and four seconds earlier than the day before.

20. Starry Night Lab Demo
Orbit A: if the sidereal day is switched to day, this view shows the background stars moving. Notice that each step forward changes only the day, but not the time. This shows a solar day (24 hours). The additional 4 minutes and 4 seconds of rotation causes the night sky and the Sun to shift approximately 1 degree to the east.

21. Starry Night Demo Chapter 5 (Seasons and the Length of Day)
View over S Pole
What is the difference between the north ecliptic pole and the north celestial pole?
What is the angular separation of these two poles?
View from Sun Centre
Why is the Earth’s axis pointing in different directions during the course of the year?
How does this affect the angle of sunlight striking the planet?

22. Starry Night Lab Demo
In Ch. 3 (diurnal motion), the sky appears to rotate from east to west.
Diurnal A: this view reinforces that the Sun, stars, and planets appear to move east-west (actually, in arcs).
Diurnal motion is caused by the rotation of the Earth.

23. Starry Night Demo Ch. 3 Lab (Diurnal Motion)
Seattle view: stars and planets rise at the same angle as the Sun rose. The apparent motion of the sky is an illusion caused by the…?
rotation of the Earth.

24. Questions
Describe what is observed in the sky during the Earth’s diurnal motion.
Explain the difference between the sidereal day and the mean solar day.
Why does the night sky appear to be different each night when observing at the same time?
Why does the night sky look the same when you observe it after one full sidereal day?
If you want to find a particular star in the exact location you observed it on the previous night, when should you look for it?

25. 30. What is the insolation angle and how does it influence the surface temperatures and seasons on Earth?