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

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

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!!

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

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

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.

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

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

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?

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

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)

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

Angles

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’

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’.

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

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

Circumpolar stars – never set

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.

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.

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

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

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?