The Sun’s Motion in the Sky
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 starcharts
Place the two rectangular charts
side by side
Locate the Celestial Equator
Locate the Ecliptic
The dates written along the Ecliptic show
the apparent location of the Sun
throughout the year.
• Ecliptic = The Path the Sun
Travels
• Zodiac = The Constellations
the Sun Travels Through
The Solstices and Equinoxes are
both:
• Specific points on the celestial sphere.
• Specific instants during the year when the
Sun is exactly at those points.
• Solstice = Sun Stops –
compare “Armistice”
• Equinox = Equal night and 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  Sun
travels just about 1 degree per day
against the celestial sphere.
• In the course of 1 year, the sun makes 1
fewer revolution than the stars do.
24 hours x 365/366 =
23 hours and 56 minutes
• 24 hours is a solar day
• 23 hours 56 minutes is a sidereal day
The Seasons
The sun goes around the
celestial sphere in 1 year.
This means the seasons
are connected to where the
sun is on 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 ALONG
THE ECLIPTIC CAUSES THE
SEASONS!
• Two effects:
• In winter, the sun is above the horizon for
a smaller fraction of the day
• In winter, even when it is up, the sun hits
the ground at a lower angle angle.
Note that in winter – same amount
of light is spread out over greater
area
Practical Application in Architecture
The Sun’s Motion viewed from the
Equator or the Pole
• Tropics = Anywhere you can stand on the
Earth’s surface and have the Sun directly
overhead (i.e. not Wisconsin).
• Arctic and Antarctic Circles = If you are
anywhere closer to the pole, the Sun will
not rise at least one day of the year.
Person Standing at the Pole:
Why are some years leap years?
Year – time for Sun to return to
the same spot on the ecliptic:
• Roughly 365.25 days long.
• If you didn’t add a leap year, time of
spring equinox would drift by 1 day every
four years. In 120 years, all the months
would be 1 month earlier (e.g. “May”
would occur in what we call “April”)
• Even leap year system is not
perfect: Year is actually 365.242
189 67 days long
• Gregorian Calendar reform:
• Thursday, 4 October 1582 was
followed by Friday, 15 October
1582 in many Catholic
countries.
• Century years (e.g. 1900) are
not leap years unless they are
evenly divisible by 400 (e.g.
2000)
• “William III of England arrived at Brixham in England on 5
November (Julian calendar), after setting sail from the
Netherlands on 11 November (Gregorian calendar).
• Shakespeare and Cervantes apparently died on exactly the
same date (23 April 1616), but in fact Cervantes predeceased
Shakespeare by ten days in real time (for dating these events,
Spain used the Gregorian calendar, but Britain used the Julian
calendar). “1
•
1Wikipedia: 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