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

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

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)

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

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

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

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

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 10 8 m/s

For example How far will light travel in an hour? Distance = velocity * time Distance = 3 x 10 8 m/s * 60 s/minute * 60 minute/hr Distance = x 10 8 m = 1.08 x m

Kepler’s 3 rd Law More distant planets orbit the Sun at slower average speeds, obeying the precise mathematical relationship p 2 = a 3 where p is a planet’s orbital period in years and a is the average distance from the Sun in astronomical units.

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

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 p 2 = a 3 = = p = √140.6 = 11.9 years

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

The calculation p 2 = a 3 = = p = √0.064 = 0.25 years

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

Everybody okay?

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

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

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

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”

Why is this number not 24 hours?

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

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

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

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

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

PRS question

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

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

PRS question #2 Why are there leap years (years with 366 days)? A) Tropical Year is ~ 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

Answer A) Tropical Year is ~ days

Spring Equinox

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 days So by the 1500s, the spring equinox had changed by 13 days

Pope Gregory Wanted to return the spring equinox to the same date So in 1582, the Pope decreed that the day after Oct. 4 th would be Oct. 15 th 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

Celestial Coordinates Declination is like latitude Right Ascencion is like longitude

Questions?