1. Bellwork 9/2
Describe what you learned in your lab yesterday in 15 words or less.

2. Basics of Celestial Navigation - stars
Coordinate systems
Earth based – latitude and longitude
Observer based – azimuth and altitude
Celestial – declination and right ascension
Motions of the stars in the sky and night sky observations
Major star groupings

3. Comments on coordinate systems
All three are basically ways of describing locations on a sphere – inherently two dimensional
Requires two parameters (e.g. latitude and longitude)
Reality – three dimensionality
Height of observer
Stars at different distances
What you see in the sky depends on
Date of year
Time
Latitude
Longitude
Which is how we can use the stars to navigate!!

4. When you look at the sky
Stars: celestial object that is self-illuminated due to fusion, twinkle
Constellation: formal name for a grouping of stars (88 total)
Example: Ursa Major
Asterism: common name for a grouping of stars
Example: Big Dipper
Planets: object that orbits a star and reflects light; don’t usually twinkle
Seeing: the stability of the sky
Bad = lots of twinkle
Observation: to look at something and take detailed information about it

5. Finding Polaris from the big dipper: Example computer!!
Schedar
Cassiopeia
Polaris
Dubhe
Big dipper/Ursa major

6. Celestial Sphere
The celestial sphere is an imaginary sphere that surrounds the earth and is used to discuss star locations
The celestial sphere appears to rotate around the earth, thus most stars rise and fall daily.

7. Altitude-Azimuth coordinate system (Horizon System)
Based on what an observer sees in the sky, the horizon, and the directions (NSEW)
Altitude = angle above the horizon to an object (star, sun, etc)
(range = -90o to 90o)
Zenith = point directly above the observer (+90o)
Nadir = point directly below the observer (-90o) – can’t be seen
Horizon = plane (0o) where the sky meets the earth
Azimuth = the cardinal directions or the circle around you
(range = 0o to 360o)
North = 0 degrees
East = 90 degrees
South = 180 degrees
West = 270 degrees

8. Point of view of the observer leads to problems:
Coordinates change when you change location
Coordinates of stars change as they move across the sky
Requires time and location for other people to determine the stars you were looking at

9. Bellwork: Why do stars appear to rise in the east and set in the west
Bellwork: Why do stars appear to rise in the east and set in the west? What does this mean for when you read a star map?

10. Equatorial System- based on latitude and longitude of earth extended to the sky
North celestial pole = point in sky directly above north pole
on earth (i.e. zenith of north pole)
South celestial pole =zenith of south pole on earth
Celestial equator – circle
surrounding equator on earth
Ecliptic – path followed
by the sun, moon, and planets through the sky over the course of
the year
Meridian: the imaginary line that extends from the NCP through the zenith to the SCP

11. Declination – latitude of stars, positive
going north (north celestial pole = + 90o), negative going
south (south celestial pole = - 90o)
This means you can determine latitude on earth by measuring the angle to Polaris
Right ascension (RA) – longitude of stars
RA – typically expressed as a time going east – 0 to 24 hours, 0h is the vernal equinox
Horizon (est)
Min. star height
Polar distance =
(90o – Declination)

12. Declination “maps” onto latitude –
At some point a star of a given
declination will pass over the zenith
at a point on the earth at its corresponding latitude.
This happens once every
24 hours…a sidereal hour maps declination perfectly onto the latitude of earth btw

14. Angles

15. Objects with Large Angular Sizes (roughly to scale)
Sun, 30’.
Andromeda Galaxy (M31)
180’ x 63’.
Orion Nebula (M42), 85’ x 60’.
M54, extragalatic
globular star cluster, 12’
Moon, 30’.
Pleiades, open star cluster, 180’.
M4, globular star cluster, 36’

16. More Examples Ring nebula, planetary nebula, 1.4’ x 1’. Crab Nebula
Supernova
remnant,
6’x4’.
Io, Jovian satellite, 1”.
Polaris A’s apparent size = 0.002”.
Polaris A to Polaris Ab is 0.2”;
Polaris A to Polaris B is 20”;
Polaris A to Dubhe ≈ 30°.
Hubble Deep Field, ≈ 1.5’.

17. How stars move through the sky
Stars seem to move across the sky at 15o per hour (4 minutes per degree)
It’s actually a bit faster because the earth is moving too, but no worries
Some stars are “circumpolar” – never set

18. Rising/setting angle is (90o – Latitude) due
east/west – along celestial equator
Angles are smaller the further N/S one goes θ

19. Circumpolar stars – never set

21. Relation between Azimuth, Latitude and Declination of
rising and setting stars
Where Rz = rising azimuth
d = declination
L = Latitude
So – at equator, L=0, cos(L) = 1, rising azimuth is the
declination of the star – exploited by Polynesians in
star compasses (near the equator cos(L) close to 1)
Can use this to find latitude, if you’re willing to do the
math, and find the azimuth of a rising star, knowing
the star’s declination.

22. Some star groupings
If you can locate stars and know the declination you can find your latitude.
With a watch, and SHA (or “stellar longitude”), you can find your longitude (must know date).
Clustering into constellations and their stories help locate stars by name.

23. Vega (Decl = 39oN) and Deneb (Decl = 45o), Altair is 9o
Summer triangle, northern cross (Cygnus)…are part of Lyra and Aquila too
Deneb
Vega
Summer
Triangle
Cygnus/
Northern
Cross
Altair
Vega (Decl = 39oN) and Deneb (Decl = 45o), Altair is 9o

24. Big Dipper as Star Guide
Oher end of the ladel point to Regulus
Handle curves to Arcturus then Spica

25. Bellwork:
You’re lost in the woods without a watch or phone. How can you figure out how much daylight you have left to build a shelter and fire?