Coordinate System&

Time/CalendarASTR 3010

Lecture 3

Textbook Chap. 3

Coordinate Systems• To describe an event in space-time• Steps of defining a spatial coord. system

1. locate the origin2. define fundamental plane3. choose the reference point4. choose signs of axes

• In astronomy, distance is superfluous most times two angles are enough to describe a point in space.

Altitude-azimuth system • aka, Horizontal coord. systemo fundamental planet=horizon,

reference point=north pointo azimuth : from north point to east, 0

– 360degrees.o altitude = h, elevation, -90 to +90

degrees.o zenith: right above the observer,

h=90 deg.o zenith angle = 90 - h

Equatorial System

o fundamental plane = celestial equator, reference point = vernal equinoxo right ascension = alpha = RA, 0h to 24ho declination = delta = Dec, -90 to +90 degreeso hour circle = great circle of constant RA, or great great circle that passes through

North Pole.

Precessiono Precession = rotation of the Earth spin axis (period=26,000 yrs 50 arcsec/yr)

vernal equinox is marching east by 50 arcsec per yearo B1950 and J2000 coordinateso International Coordinate Reference System (ICRS): reference point was chosen to

a fixed point on the celestial sphere that is close to that of J2000 epoch. Precession free!

Relationship among latitude, altitude, and declinationo circumpolar stars?o altitude of NP = latitude of an observero Meridian : great circle that passes through zenith and N.P.o transit : when an object crosses the Meridian (maximum altitude)o hour angle of an object = RA of Meridian – RA of the objecto local sidereal time = RA of Meridian

Ecliptic coordinate system• fundamental plane = ecliptic, reference point = vernal equinox• useful to describe solar system objects because they are all confined within

±10 degrees from the ecliptic.

Galactic coordinate system• fundamental plane = Galactic disk, reference point = toward the Galactic center• longitude (l) and latitude (b)

Solar Time• Sidereal time = RA of an object in transito Earth’s rotation rate relative to distant starso sidereal day = 23.93447 hours

• Solar time = Time tracked by the Sun (local noon is when the Sun transits)o solar time = RA of the Sun + 12 hourso solar day = 24 hours

Apparent Sun’s annual motion across the sky• analemma• Mean solar time : using a fictitious mean Sun that is moving at a constant

speed (i.e., on a perfect circular orbit) : solar time and mean solar time can differ upto 16min

Solar year (tropical year)• the length of the time that Sun returns to the same position in its orbit

relative to the Earth (i.e., vernal equinox to vernal equinox)• 365.242581 days• Civil calendar (Gregorian calendar) = 365 days.• To compensate the differenceo every 4th year, add one day in February (Leap Day)

365.242581 – 365.0 = 0.242581 days 0.242581×4 = 0.970323 days but 1 whole day was added over 4 years,

0.029676 day is too long! over 400 years, 2.9676 days too long Then, let’s remove three leap years over 400 years Among those leap years

(divisible by 4), if a year is divisible by 100 but not by 400, it is no longer a leap year (1900 is not a leap year but 2000 is).

over 400 years, about 2790 seconds too short. Add +1 second occasionally (leap second).

Julian Date• Continuous count of days since 4713 BC Jan 1, 12PM• Useful to denote the epoch of astronomical observation

• Modified Julian Date (MJD) = JD – 2400000.5, most commonly used in astronomy (introduced by SAO to track Sputnik using 18bit number).

Visibility of an object

(Q) You plan to observe celestial objects tonight (August 21) at Athens, GA (34°N). If you can point your telescope down to h=30°, what are ranges of Right Ascension and Declination for observable objects? Assume that you the length of night is 8 hours and you will only observe objects when they transit.

In summary…

Important Concepts• various coordinate systems• Time system

• Visibility of an object for an observer

Important Terms• zero magnitude flux• great circle• meridian• hour circle• zenith, north point• transit• hour angle• etc.

Chapter/sections covered in this lecture : Chap 3