Celestial Sphere Remember that we are on the INSIDE of the Our lack of depth perception when we look into space creates the illusion that Earth is surrounded by a celestial sphere. In reality, stars that appear very close together in our sky may actually lie at very different distances from Earth. Remember that we are on the INSIDE of the sphere (on Earth) looking out!

Points on the Celestial Sphere North and south celestial poles Celestial equator REMEMBER: These are points /lines on the celestial sphere and NOT on the Earth From now on: equator = celestial equator

The Dome of the Local Sky Zenith Nadir Horizon Meridian Transit

Horizon coordinate system Horizon coordinate system - coordinates are measured with respect to horizon - change with time and depend on observer Azimuth: 0 to 360 degrees around horizon from north towards east 0° = North, 90 ° = East, 180 ° = South, 270 °= West Altitude: 0 to 90 degrees up from horizon 0 ° = Horizon, 90 ° = Zenith

Ecliptic Plane Plane containing the Sun and planets Ecliptic is tilted 23.5° with respect to the Equator Eclipses can only occur when the moon crosses this plane

Zodiac Constellations Ecliptic: The Sun's apparent annual path among the constellations Zodiac Constellations The constellations on the celestial sphere through which the ecliptic passes Origin of Astrology (Zodiac Sign)

Cardinal Points on the Ecliptic Vernal Equinox Sun rises due East and sets due West Length of day = length of night = 12 hours Summer Solstice Sun is highest in the sky (this is why it’s so hot during summer) Autumnal Equinox Winter Solstice Sun is lowest in the sky (this is why it’s so cold during winter)

Don’t confuse RA with time on your watch! Equatorial coordinate system - coordinates fixed on the celestial sphere - time and observer independent declination (dec)‏ Analogous to latitude, but on the celestial sphere; it is the angular north-south distance between the celestial equator and a location on the celestial sphere. Measured in degrees: 0 ° to 90 ° – north from celestial equator 0 ° to -90 ° – south from celestial equator right ascension (RA)‏ Analogous to longitude, but on the celestial sphere; it is the angular east-west distance between the vernal equinox and a location on the celestial sphere. Measured in units of time: hours, minutes, seconds 0 h – 24 h from Vernal Equinox towards east Ex. Sirius has RA = 6 h 45 m OR 6:45 Don’t confuse RA with time on your watch!

Equatorial coordinate system Comparing latitude and longitude to declination and right ascension

RA and Dec of the Cardinal Points on the Ecliptic Vernal Equinox Sun appears on March 21 RA = 0h Dec = 0˚ Summer Solstice Sun appears on June 21 RA = 6h Dec = 23.5˚ Autumnal Equinox Sun appears on Sept. 21 RA = 12h Dec = 0˚ Winter Solstice Sun appears on Dec. 21 RA = 18h Dec = -23.5˚

RA and Dec of the Cardinal Points on the Ecliptic 23.5° Vernal Equinox Sun appears on March 21 RA = 0h Dec = 0˚ Summer Solstice Sun appears on June 21 RA = 6h Dec = 23.5˚ Autumnal Equinox Sun appears on Sept. 21 RA = 12h Dec = 0˚ Winter Solstice Sun appears on Dec. 21 RA = 18h Dec = -23.5˚ Equator Declination 0 h 6 h 12 h 18 h 24 h Ecliptic -23.5°

Example: where is Vega? Its declination tells us that it is 38°44′ north of the celestial equator. We can interpret its right ascension in two ways: As an angle, it means Vega is about 279° east of the vernal equinox As a time, it means Vega crosses the meridian about 18 hours 35 minutes after the spring equinox.

Understanding Local Skies 3 classes of stars: circumpolar north - always visible circumpolar south - never visible rising and setting

Understanding Local Skies The sky at the North Pole.

Understanding Local Skies The sky at the equator

Understanding Local Skies The sky at 40˚N latitude.

Understanding Local Skies The sky at 30˚S latitude.

The altitude of the celestial pole in your sky is equal to your latitude. Everything in the sky rotates around the north celestial pole

Sidereal Time Sidereal time 1) Time measured according to the position of stars in the sky rather than the position of the Sun in the sky. 2) How long ago the vernal equinox has transited 3) It’s the Right Ascension of ANY transiting star.

Problem 1 What is the Sidereal Time at noon, December 21? Sidereal Time = RA of transiting Star What star is transiting at noon? Answer: Sun What is significant about Dec. 21? Answer: Winter Solstice: Sun has an RA of 18 hours Therefore, the Sidereal Time at noon on Dec. 21 is 18:00

Problem 2 Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°) Equator Problem 2 Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°) from an observatory at latitude 50° N? Set up a picture: Do the math: The horizon is 90° from your zenith Zenith has a dec = your lat = 50° The lowest point in the sky that you can see has a dec of: 50 ° - 90 ° = - 40 ° 3. The star is 5 ° below the Horizon…so we can’t see it Zenith Dec = 50° Equator NCP Lat = 50° Horizon Dec = -40° Celestial Sphere Earth Kapteyn’s Star Dec = -45° NCP Horizon 50° 40° -5° Equator

Problem 4 A star lies 15 degrees due east of zenith at 10 PM; when will it transit? Recall that 1 hour is equal to 15 degrees. Why is this? Answer: Earth rotates 360 degrees in 24 hours  360 / 24 = 15 degrees per hour So we know the star will transit 1 hour before or after 10 PM. Since the star is east of the meridian, it hasn’t yet transited. (all stars rise in the east and set in the west as time passes) Therefore, the star will transit in one hour: 11 pm Zenith East West

Problem 5 What is the maximum altitude and the azimuth of the sun at noon, September 21 in Storrs, CT.? Set up a picture: Do the math: Storrs has a lat of 42 ° N The horizon is 90° from your zenith Zenith has a dec = your lat = 42° The lowest point in the sky that you can see has a dec of: 42 ° - 90 ° = - 48 ° The sun has a dec of 0 ° on the Autumn Equinox It’s altitude is then: 48 ° Since it is transiting at noon in the south, it’s azimuth must be 180 ° Zenith Dec = 42° Equator NCP Lat = 42° Horizon Dec = -48° Celestial Sphere Earth Sun Dec = 0° NCP Horizon 42° 48° Equator 48 Degrees

Naming The Stars The brightest stars have had proper names for thousands of years. Typically from Arabic Islamic astronomers produced many detailed star charts during the middle ages. Also many stars have names that are from the Greek Greek astronomers also produced many star charts.

Naming the Stars Proper names are often a literal description of the star’s location in the constellation: Betelgeuse – “Shoulder of the giant” Rigel – “Foot” Deneb – “Tail” Procyon – “Before the dog” Algol – “Eye of the ghoul” Star proper names also sometimes describe the star. Sirius – “Scorching” Antares – “Rival of Mars” Kochab – “Star”

Scam Alert! NO organization has been given official legal power to name the stars. Astronomers consider the names designated by the International Astronomical Union official, but the IAU has no true legal authority to name stars (or demote planets, for that matter). Companies that sell star names as gifts are a SCAM!

Bayer Letter Names Johann Bayer Bavararian lawyer Published a sky atlas: Uranometria. Assigned lower case greek letters to brighter stars in each constellation. Usually assigned in rough order of brightness. Alpha = brightest star. Beta = second brightest Gamma = third brightest And so on… Astronomers continue to use these “Bayer letters”

Bayer Letter Names A star’s Bayer Letter Name is: It’s greek letter first… …followed by the possessive form of the constellation name. Example: The star Rigel Kentarus is the brightest star in the constellation Centarus: Alpha Centauri

The Magnitude Scale If Star A is has a magnitude this much less than Star B… …then Star A is this many times brighter than Star B. 1 2.512× 2 6.310× 3 15.85× 4 39.82× 5 100×

The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42 Full moon: mv = -12.5 Sun: mv = -26.5