The Sun’s Motion in the The Sun’s Motion in the SkySky

Where does sunset occur in December?

The Setting Sun – “The Sun doesn’t like to set”

Map and Unwrap the region on the Celestial sphere where the Sun travels

This does to the Celestial Sphere what a Mercator projection does to

a globe

Solstices and Equinoxes mark the beginnings of their seasons

Get out your SFA starchartsGet out your SFA starchartsPlace the two rectangular Place the two rectangular charts side by sidecharts side by side

Locate the Celestial EquatorLocate the Celestial Equator

Locate the EclipticLocate the Ecliptic

The dates written along the Ecliptic show The dates written along the Ecliptic show the apparent location of the Sun the apparent location of the Sun throughout the year. throughout the year.

•Ecliptic = The Path the Sun Ecliptic = The Path the Sun TravelsTravels

•Zodiac = The Zodiac = The Constellations the Sun Constellations the Sun Travels ThroughTravels Through

The Solstices and Equinoxes The Solstices and Equinoxes are are both:both:• Specific points on the celestial Specific points on the celestial

sphere. sphere.

• Specific instants during the year Specific instants during the year when the Sun is exactly at those when the Sun is exactly at those points. points.

•Solstice = Sun Stops – Solstice = Sun Stops – compare “Armistice”compare “Armistice”

•Equinox = Equal night and Equinox = Equal night and day (12 hours each)day (12 hours each)

Ecliptic (in red) on Celestial Sphere

What time of day is it?1.Morn, 2.Noon, 3.Eve, 4.Night

What is the season?1 = Sum., 2=Fall, 3=Win., 4=Spr.

• There are 365 days in a year There are 365 days in a year Sun Sun travels just about 1 degree per day travels just about 1 degree per day against the celestial sphere.against the celestial sphere.

• In the course of 1 year, the sun In the course of 1 year, the sun makes 1 fewer revolution than the makes 1 fewer revolution than the stars do.stars do.

24 hours x 365/366 =24 hours x 365/366 =23 hours and 56 minutes23 hours and 56 minutes

• 24 hours is a solar day24 hours is a solar day

• 23 hours 56 minutes is a sidereal day23 hours 56 minutes is a sidereal day

TheThe SeasonsSeasonsThe sun goes around The sun goes around

the celestial sphere in 1 the celestial sphere in 1 year. This means the year. This means the

seasons are connected seasons are connected to where the sun is on to where the sun is on the celestial sphere.the celestial sphere.

Summer Solstice (June 20)(line shows daily path of Sun)

At the Spring and Fall Equinoxes(March 20 and September 20)

Winter Solstice(December 21)

THE MOTION OF THE SUN THE MOTION OF THE SUN ALONG THE ECLIPTIC CAUSES ALONG THE ECLIPTIC CAUSES THE SEASONS!THE SEASONS!

• Two effects:Two effects:

• In winter, the sun is above the horizon In winter, the sun is above the horizon for a smaller fraction of the dayfor a smaller fraction of the day

• In winter, even when it is up, the sun hits In winter, even when it is up, the sun hits the ground at a lower angle angle.the ground at a lower angle angle.

Note that in winter – same amount of light is spread out over greater

area

Practical Application in Practical Application in ArchitectureArchitecture

The Sun’s Motion viewed from The Sun’s Motion viewed from the Equator or the Polethe Equator or the Pole

• Tropics = Anywhere you can stand on Tropics = Anywhere you can stand on the Earth’s surface and have the Sun the Earth’s surface and have the Sun directly overhead (i.e. not Wisconsin). directly overhead (i.e. not Wisconsin).

• Arctic and Antarctic Circles = If you are Arctic and Antarctic Circles = If you are anywhere closer to the pole, the Sun will anywhere closer to the pole, the Sun will not rise at least one day of the year.not rise at least one day of the year.

Person Standing at the Pole:

Why are some years leap Why are some years leap years?years?

Year – time for Sun to return Year – time for Sun to return to the same spot on the to the same spot on the ecliptic:ecliptic:• Roughly 365.25 days long. Roughly 365.25 days long.

• If you didn’t add a leap year, time of If you didn’t add a leap year, time of spring equinox would drift by 1 day spring equinox would drift by 1 day every four years. In 120 years, all every four years. In 120 years, all the months would be 1 month earlier the months would be 1 month earlier (e.g. “May” would occur in what we (e.g. “May” would occur in what we call “April”)call “April”)

• Even leap year system is not Even leap year system is not perfect: Year is actually perfect: Year is actually 365.242 189 67 days long365.242 189 67 days long

• Gregorian Calendar reform: Gregorian Calendar reform:

• Thursday, 4 October 1582 Thursday, 4 October 1582 was followed by Friday, 15 was followed by Friday, 15 October 1582 in many October 1582 in many Catholic countries. Catholic countries.

• Century years (e.g. 1900) Century years (e.g. 1900) are not leap years unless are not leap years unless they are evenly divisible by they are evenly divisible by 400 (e.g. 2000)400 (e.g. 2000)

• ““William III of England arrived at Brixham in England on William III of England arrived at Brixham in England on 5 November (Julian calendar), after setting sail from the 5 November (Julian calendar), after setting sail from the Netherlands on 11 November (Gregorian calendar).Netherlands on 11 November (Gregorian calendar).

• Shakespeare and Cervantes apparently died on exactly Shakespeare and Cervantes apparently died on exactly the same date (23 April 1616), but in fact Cervantes the same date (23 April 1616), but in fact Cervantes predeceased Shakespeare by ten days in real time (for predeceased Shakespeare by ten days in real time (for dating these events, Spain used the Gregorian dating these events, Spain used the Gregorian calendar, but Britain used the Julian calendar). “1calendar, but Britain used the Julian calendar). “1

• 1Wikipedia: Gregorian Calendar1Wikipedia: Gregorian Calendar

Exercise 4: Planisphere -- Sun

Exercise 5: Hours of the Day around the World

Sun’s Motion through the Stars – Caused by Earth’s motion

Day of the year tells you exactly where the earth is on its orbit.

Ex 6: Earth in Orbit around the Sun

Ex 7: The constellations throughout the year