Lecture 2: Astronomy Origins

Question 1

Angles and Separation in the Sky

Answer 1

• For two objects in the sky: Arc Length = Distance * Angle between them
• ## For small enough angles, the arc length is approximately equal to the distance between the objects

Question 2

Earth’s Motion around the Sun

Answer 2

• Sun appears in a different position relative to the stars every day.
• Constellations along the Sun’s path are called the zodiac
• The ecliptic is the Sun’s path through the sky, while the equator is the Earth’s axis of rotation, the angle between the two is 23.5 degrees
• Planets’ paths in the sky are very similar to the ecliptic

Question 3

Position of sun throughout the year

Answer 3

• March 22 is the vernal equinox and is when the sun is directly above the equator and right ascension is 0.

Question 4

Time Measurement

Answer 4

• Sidereal time is according to the stars
• Synodic time is according to the sun (also called solar time)
• Synodic day is 4 min longer than sidereal day
• Apparent solar time is time measured by sun’s position in the sky
• Mean solar time is the average length of a solar day
• A tropical year is the time from one spring equinox to the next

Question 5

The moon and it’s phases

Answer 5

• A month is a time based on the position of the moon relative to the sun, as viewed by the earth. This is SYNODIC time

Question 6

The calendar

Answer 6

• Synodic month is 29.53 days while sidereal month is 27.32 days
• Roman calendar had a 13th month every 3rd year
• Julian calendar had 12 months with 30 or 31 days each and a leap years every 4th year, started January 1, 45 BCE
• Gregorian calendar is what we currently use, and skipped 10 days in October of 1582 to balance calendar mismatch

Question 7

Eclipses

Answer 7

• A lunar eclipse is when the moon passes in the earth’s shadow and disappears, can only happen at full moon
• A solar eclipse is when the earth passes through the moon’s shadow, so the sun is eclipsed, can only happen at new moon

Question 8

Stone-age astronomy

Answer 8

• Used to mark seasons and special times of the year
• Examples include Stonehenge, the pyramids, mayan-peru, medicine wheels, and sundials

Question 9

Ancient greek astronomy

Answer 9

• The world was a fixed unchanging sky with 7 planets, the sun, the moon, mercury, Venus , mars, Jupiter and Saturn
• planet = “wanderer”
• Pythagoras, Aristotle, Aristarchus of Samos, Eratosthenes, Hipparchus, Ptolemy, and Philolaus,

Question 10

Earth-centric vs Helio-centric

Answer 10

• Epicycles invented to explain retrograde motion in a geocentric universe
• Opposition and Conjunction