1. Astronomical Time Periods

2. Sidereal Day and Solar Day
Sidereal Day = The time it takes a STAR to circle the Earth once.
Solar Day = The time it takes the SUN to circle the Earth once.
Why are they different?

3. Sidereal Day and Solar Day
Sidereal day covers 360º of rotation:
23 h 56 m s
Solar day covers ~361º of rotation
24 h ± ~25 s
It varies throughout the year…

4. It varies throughout the year – WHY?
Kepler’s second law…

5. When is a solar day the longest?
A) January
B) April
C) July
D) October
E) Solar days are all the same – sidereal days vary throughout the year

6. Sidereal Month and Synodic Month
Sidereal month = The time it takes the Moon to go 360º around Earth.
27⅓ days
Synodic Month = The time from New Moon to New Moon.
29½ days

7. The Moon rotates on its axis once in a …
A) Synodic month
B) Sidereal month
C) Both are correct

8. Important planetary positions…
…you should know
Planets outside Earth’s orbit
All planets orbit in the same direction
Planets inside Earth’s orbit

9. Mercury’s position in the sky at the same time night after night.
You can see it moving around its orbit as it “catches up” to Earth.

10. Venus’s position in the sky at the same time night after night.
Venus moves slower, around a larger orbit, than Mercury, so its path is longer

11. Just because Mercury/Venus go through inferior conjunction doesn’t mean they pass in front of the Sun.

12. “Transit” Transits of Mercury happen every 3 – 13 years.
(May 9, 2016; Nov 11, 2019)
Transits of Venus are much more rare…
“Transit”

13. Transits of Venus:
1631, 1639, 1761, 1769, 1874, 1882, 2004,
2012 … 2117, …

14. Noon-to-noon (“solar day”) is not always exactly 24 hours
Apparent Solar Time
Noon-to-noon (“solar day”) is not always exactly 24 hours

15. So our clocks are set to “Mean Solar Time”
(“mean” = “average”)

16. These are both “local times”
People in Pueblo would have their clocks set differently from people in Alamosa.
Not a problem when travel by wagon took at least a day…

17. But when the railroads shrank the distances between towns, something had to be done!

18. Railroad companies agreed to break the U.S. into 4 zones,
November 18, 1883
Railroad companies agreed to break the U.S. into 4 zones,
setting all clocks in each zone to the same time
International consensus for global time zones was reached in 1884

19. International Dateline
MST = UT – 7
Universal Time
International Dateline
Daylight Saving Time

20. Calendars: Tied to the Equinox
Egyptian: B.C.
365 days in a year
Vernal equinox drifted 1 day LATER every 4 years

21. Calendars: Tied to the Equinox
Julian: Julius Caesar - 46 B.C.
Every 4th year has 366 days
It was too much correction:
In 46 B.C, Spring Equinox on March 24
By 1500’s, Spring Equinox on March 11

22. Calendars: Tied to the Equinox
Gregorian: Pope Gregory XIII – 1582
1. Skipped from Oct. 4  Oct. 15
This set the spring equinox to March 21
2. Every 4th year has 366 days YES NO
EXCEPT for centuries not divisible by 400:

23. Date and Time of Spring Equinox on Consecutive Years
UT dd hh mm
Date and Time of Spring Equinox on Consecutive Years

24. Why do we have leap years?
Because the Earth’s precession causes the spring equinox to drift by 6 hours every year.
Because the Earth takes almost 6 hours more than 365 days to complete one full revolution about the Sun.
Because the Earth takes almost 6 hours less than 365 days to complete one full revolution about the Sun.
To make up for an inadequacy in our modern calendar.
To make up for the slowing of the Earth’s rotation.

25. Astro-Cash Cab! Emily Sowards Sawyer Robison Isabel Rodriguez
Jesse Klene
Brittany Laske

26. 1) Which statement is false?
A) A solar day is always longer than a sidereal day.
B) The length of a sidereal day varies over the course of a year.
C) A synodic month is longer than a sidereal month.
D) Only inner planets have two kinds of conjunctions.

27. 2) Which calendar do we use today?
Egyptian calendar
Julian calendar
Gregorian calendar
Mayan calendar

28. 3) True or False?
When Venus is at Greatest Eastern Elongation, it is in the eastern sky at sunrise.

29. 4) What technological advancement led to the development of time zones?