Presentation on theme: "Coordinate System & Time/Calendar"— Presentation transcript:
1 Coordinate System & Time/Calendar ASTR 3010Lecture 3Textbook Chap. 3
2 Coordinate Systems To describe an event in space-time Steps of defining a spatial coord. systemlocate the origindefine fundamental planechoose the reference pointchoose signs of axesIn astronomy, distance is superfluous most times two angles are enough to describe a point in space.
3 Altitude-azimuth system aka, Horizontal coord. systemfundamental planet=horizon, reference point=north pointazimuth : from north point to east, 0 – 360degrees.altitude = h, elevation, -90 to +90 degrees.zenith: right above the observer, h=90 deg.zenith angle = 90 - h
4 Equatorial Systemfundamental plane = celestial equator, reference point = vernal equinoxright ascension = alpha = RA, 0h to 24hdeclination = delta = Dec, -90 to +90 degreeshour circle = great circle of constant RA, or great great circle that passes through North Pole.
5 PrecessionPrecession = rotation of the Earth spin axis (period=26,000 yrs 50 arcsec/yr) vernal equinox is marching east by 50 arcsec per yearB1950 and J2000 coordinatesInternational 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!
6 Relationship among latitude, altitude, and declination circumpolar stars?altitude of NP = latitude of an observerMeridian : great circle that passes through zenith and N.P.transit : when an object crosses the Meridian (maximum altitude)hour angle of an object = RA of Meridian – RA of the objectlocal sidereal time = RA of Meridian
7 Ecliptic coordinate system fundamental plane = ecliptic, reference point = vernal equinoxuseful to describe solar system objects because they are all confined within ±10 degrees from the ecliptic.
8 Galactic coordinate system fundamental plane = Galactic disk, reference point = toward the Galactic centerlongitude (l) and latitude (b)
9 Solar Time Sidereal time = RA of an object in transit Earth’s rotation rate relative to distant starssidereal day = hoursSolar time = Time tracked by the Sun (local noon is when the Sun transits)solar time = RA of the Sun + 12 hourssolar day = 24 hours
10 Apparent Sun’s annual motion across the sky analemmaMean 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
11 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)daysCivil calendar (Gregorian calendar) = 365 days.To compensate the differenceevery 4th year, add one day in February (Leap Day)– = days×4 = days but 1 whole day was added over 4 years, day is too long! over 400 years, days too longThen, 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).Mean solar day is changing due to (1) slow down of Earht’s rotation, (2) other planets’ influence, etc. since this calendar year is measured in terms of mean solar day, precise calculation needs to take the variation of mean solar day which is quite stochastic. Lean seconds are announced only in 6 months advance…
12 Julian Date Continuous count of days since 4713 BC Jan 1, 12PM Useful to denote the epoch of astronomical observationModified Julian Date (MJD) = JD – , most commonly used in astronomy (introduced by SAO to track Sputnik using 18bit number).
13 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.
14 Chapter/sections covered in this lecture : Chap 3 In summary…Important ConceptsImportant Termsvarious coordinate systemsTime systemVisibility of an object for an observerzero magnitude fluxgreat circlemeridianhour circlezenith, north pointtransithour angleetc.Chapter/sections covered in this lecture : Chap 3
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