1. Celestial Coordinates
Celestial Navigation

2. Positions on the Celestial Sphere
On the Earth:
Latitude  north – south
Longitude  east – west
On the Celestial Sphere:
Declination north – south (degrees)
Right Ascension east – west (hours, min, sec)

3. What is the Sun’s right ascension on the spring equinox?
B) 90º
C) 0 hrs
D) 6 hrs
E) 12 hrs

4. What is the Sun’s declination on the (northern hemisphere) summer solstice?
B) 23.5º
C) 90º
D) –23.5º
E) –90º

5. There’s a complication…
Positions on the Celestial Sphere
On the Earth:
Latitude  north – south
Longitude  east – west
On the Celestial Sphere:
Declination north – south (degrees)
Right Ascension east – west (hours, min, sec)
There’s a complication…

6. Spring Equinox 2016

7. Spring Equinox 2516

8. Spring Equinox 2016 – High magnification
Field is 2º wide

9. Spring Equinox 2017 – High magnification
Field is 2º wide
Because the spring equinox precesses, we can talk about two different “years”

10. Sidereal year = time for Earth to go 360º around Sun w.r.t the stars
( days)
Tropical year = time for Earth to go from one Spring Equinox to the next
( days)
The tropical year is about 20 minutes shorter than the sidereal year
The tropical year is the basis for our calendars.
Because of the difference, the constellations seen in each season change over time.

11. In other words, it’s one full cycle of precession!
20 minutes a year doesn’t sound like much, but over 26,000 years, it adds up to a full year
Polaris
In other words, it’s one full cycle of precession!

12. Celestial Navigation
Determining latitude is relatively easy, day or night.
Measure the altitude of a bright star or the Sun as it crosses your meridian.
Ships would carry books with tables of the declination of bright stars, as well as
the Sun’s declination on each day of the year.

13. Longitude is much more of a problem!
In principle, it’s pretty straightforward:
Measure the time at which the Sun crosses the meridian.
For every 4 minutes late (compared to Greenwich, England) you are 1º further west.
Practically speaking, you need a very accurate clock on board so you know the current time in Greenwich, England.
At the equator, each degree is about 70 miles:
1 minute error = 17.5 miles off course
It wasn’t until the late 1700’s that mechanical clocks were developed that could keep accurate time while at sea for extended voyages

14. Astro-Cash Cab!
Demetrius
Chandley
Cale
Elizabeth
Kyle

15. It is measured in units of _______________.
1) Right ascension measures ______________ position on the celestial sphere.
It is measured in units of _______________.
A) East – West; degrees, 0 – 360º
B) North – South; degrees, 0 – 90º
C) North – South; hours, 0 – 9
D) East – West; hours, 0 – 24
E) East – West; hours, 0 – 360

16. in ____________________.
2) To determine their latitude, mariners of the 18th century had to compare the time the sun crossed their meridian to the time it crossed the meridian
in ____________________.

17. A tropical year is 20 minutes longer than a sidereal year.
3) True or False?
A tropical year is 20 minutes longer than a sidereal year.

18. 4) Why is the tropical year different from the sidereal year?