Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine tburbine@mtholyoke

1. Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.edu

2. Homework #4 • Its on OWL. • The first homework assignment has 10 questions. For each question, you will get two attempts to answer it correctly. • After your last attempt, the correct answer will be shown. • If you get all 10 questions correct, you will get one homework point. • If you get less than 10 questions correct, I will divide the number of questions you get correct by 10 to determine the fraction of a homework point that you will receive. • This is due by Thursday evening (2/10) at 11:59 pm

3. 2 Rooms for Test • Hasbrouk 20 (For people whose last names begin with letters between A and S) • Morrill Science Center (Bldg IV) room N201 (For people whose last names begin with letters between T and Z)

4. Test on Tuesday • What you should do: • Go over the lecture notes • Read the book

5. Test • 40 questions • Choices A, B, C, D, E • 1 hour and 15 minutes to complete it • No calculator • No books • No notes

6. Test Philosophy • People who have gone to class should be rewarded • People who have not gone to class should be punished

7. What the test covers • Chapter 1 • Chapter 2 • Chapter 3 • Supplemental Chapter (what I cover in class today)

8. Formulas you need to know • velocity = distance/time • Distance = velocity * time • Time = distance/velocity • If you are given two of the quantities, you can calculate the third • This equation will primarily be used for calculating the distance light travels or the time it takes for light to travel a particular distance • Speed of light = 3 x 108 m/s

9. For example • How far will light travel in an hour? • Distance = velocity * time • Distance = 3 x 108 m/s * 60 s/minute * 60 minute/hr • Distance = 10800 x 108 m = 1.08 x 1012 m

10. Kepler’s 3rd Law • More distant planets orbit the Sun at slower average speeds, obeying the precise mathematical relationship p2 = a3 where p is a planet’s orbital period in years and a is the average distance from the Sun in astronomical units.

11. Calculations • The period for the Earth to go around the Sun is 1 year • The distance of the Earth to the Sun is 1 Astronomical Unit

12. How long does it take Jupiter to go around the Sun • If Jupiter is 5.2 Astronomical Units from the Sun, how long does it take Jupiter to go orbit the Sun once • p2 = a3 = 5.23 = 140.6 • p = √140.6 = 11.9 years

13. Another example • Mercury is 0.4 Astronomical Units from the Sun. • How long does it take Mercury to orbit the sun once? • A) 1 year • B) 3 months • C) 9 months • D) 5 years

14. The calculation • p2 = a3 = 0.43 = 0.064 • p = √0.064 = 0.25 years

15. Things you need to know • Ecliptic • Zodiac • Order of the planets • Umbra, penumbra • Famous people – what did they do • Kepler’s 3 laws • Lightyear

16. Everybody okay?

17. Keeping Time • If you want to predict the positions of the planets and stars, you need to keep accurate time • What’s a day?

18. One thing that you need to know • 360 degrees in a circle

19. Types of Day • Solar Day – time it takes the Sun to make one circuit around our local sky • An average of 24 hours over the course of a year

20. Types of Day • Sidereal day – the time it takes between successive appearances of any particular star on the meridian 23 hours 56 minutes 4.09 seconds • Sidereal means • “related to the stars”

21. Why is this number not 24 hours?

22. Because the Earth is also moving in its orbit • The Earth moves about 1 degree a day in its orbit around the sun • So the Earth must rotate 361 degrees around its axis • Each 4 minutes the Earth rotates around 1 degree

24. Months • Synodic month – there is a 29.5 day cycle of phases of the Moon

26. Sidereal Month • Sidereal Month – the time it takes for the Moon to orbit the Earth once (relative to the stars) = 27.33 days

27. Year • Sidereal Year – time it takes for the Earth to complete one orbit around the stars • Tropical year- time it takes for the Earth to go from one spring equinox to the next spring equinox • The length of the Tropical Year is the basis of the modern calendar

28. Time Difference • Tropical year is 20 minutes shorter than a sidereal year • This would cause the year to be out of sync with the seasons by 1 day every 72 years • The length of the Tropical Year is the basis of the modern calendar

29. PRS question

30. PRS Question • Why is there a time difference between the sidereal year and the tropical year? • A) Conjuction • B) Precession • C) Ecliptic • D) Horizon

31. Precession • Precession changes the orientation of the axis in space but also changes the location in Earth’s orbit when seasons occur • Each year the location of the equinoxes and solstices relative to the stars shifts about 1/26,000 of the way around its orbit • (1/26000) * one year = 20 minute

32. PRS question #2Why are there leap years (years with 366 days)? • A) Tropical Year is ~365.25 days long • B) Tropical Year is ~366 days long • C) Tropical Year is ~365.1 days long • D) Tropical Year is ~365.5 days long

33. Answer • A) Tropical Year is ~365.25 days

34. Spring Equinox

35. Without a leap year • The dates of the spring equinox would move • Egyptians came up with the idea of a year having 365 days • In 46 BC, Julius Caesar decreed the adoption of a leap year (his spring equinox was March 24) • However, a tropical year is about 11 minutes short of exactly 365.25 days • So by the 1500s, the spring equinox had changed by 13 days

36. Pope Gregory • Wanted to return the spring equinox to the same date • So in 1582, the Pope decreed that the day after Oct. 4th would be Oct. 15th • This made the spring equinox March 21 • Made exceptions to the leap year rules • Leap year is skipped when a century changes unless the leap year is divisible by 400 • A year in the Gregorian calendar is almost exactly the same as a tropical year

37. Celestial Coordinates • Declination is like latitude • Right Ascencion is like longitude

40. Questions?