Stars and star names

88 official constellations

α alpha i iota ρ rho β beta κ kappa σ sigma γ gamma λ lambda τ tau δ delta μ mu υ upsilon ε epsilon ν nu φ phi ζ zeta ξ xi χ chi η eta ο omicron ψ psi θ theta π pi ω omega Constellation Possessive Aries Arietis Taurus Tauri Gemini Geminorum Cancer Cancri Leo Leonis Virgo Virginis Libra Librae Scorpius Scorpii Sagittarius Sagittarii Capricornus Capricornii Aquarius Aquarii Pisces Piscium

Betelgeuse =  - Orionis = HIP = etc…. Mintaka =  - Orionis = HIP = etc…..

Objects move daily across the sky. Why?

North pole (Right hand rule) Because we live on a ball that’s spinning!

Because we live on a ball that’s spinning: N up E WRise in the East

because we live on a ball that’s spinning: up N E

N N E W Sets in the West

So, everything appears to move from east to west. (On a daily basis)

Two kinds of day! Solar day: The time it takes the earth to rotate once with respect to the sun. Trouble: The solar day changes with the time of the year!

Mean Solar day: The average of the solar day over a year. Our clocks measure mean solar time (MST): 1 hour = 1/24 th of a mean solar day 1 min = 1/60 th of an hour 1 sec = 1/60 th of a min

Sidereal Day: time between a star’s meridian crossings 1 sidereal day = 23 hr 56 min and sec (MST) If the sidereal day is constant, why not use it to calibrate our watches? sidereal day also divided up into hour, minutes, sec (sidereal)

The Celestial Sphere

We can think of all these stars as fixed on a big celestial sphere C.S. fixed Earth spins towards East OR …

We can think of all these stars as fixed on a big celestial sphere Earth fixed C.S. spins towards West

What we see from our specific locale

The natural (local) way: Apparent S Horizon Apparent N Horizon Zenith N pole S pole Apparent horizons depend on your height, mtns etc …

The natural (local) way: Zenith N pole S pole ‘True’ (local) horizon

celestial N pole celestial S pole celestial equator celestial sphere rotates Polaris

celestial N pole celestial S pole celestial equator local horizon zenith Earth’s equator latitude

celestial N pole celestial S pole celestial equator local horizon Make Earth very small (Since everything is so far away!) zenith latitude Polaris latitude

local horizon

celestial N pole celestial S pole celestial equator local horizon celestial sphere rotates (instead of the earth) Motions of the stars relative to the fixed earth!

celestial N pole celestial S pole celestial equator local horizon zenith Blue area is below observer’s horizon

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon zenith

celestial N pole celestial S pole celestial equator local horizon Circumpolar!

celestial N pole celestial S pole local horizon: observer at earth’s equator Observer’s zenith No circumpolar

celestial N pole celestial S pole local horizon: observer at 45˚ N Observer’s zenith

celestial N pole celestial S pole local horizon: observer N pole zenith All circumpolar

Putting the sun’s position on the celestial sphere (The ecliptic) Earth’s tilted by 23 ½ °

Sun’s daily motion Sun’s yearly motion on celestial sphere

The seasons Earth’s tilted by 23 ½ ° Relative to the plane of its orbit around the sun

Axis always points in the same direction (celestial N pole)

celestial N pole (N) summer solstice ≈ June 21

celestial N pole Equinox: (N) Autumn (Autumnal) ≈ September 22 (N) Spring (Vernal) ≈ March 21

celestial N pole (N) winter solstice ≈ December 21

Why is winter cold and summer hot?

“Precession of the equinox”

The Earth isn’t a perfect sphere! Non-zero torque from sun and moon!

Spinning objects subjected to a torque precess!

Precession of the North celestial pole.