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**Coordinate System & Time/Calendar**

ASTR 3010 Lecture 3 Textbook Chap. 3

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**Coordinate Systems To describe an event in space-time**

Steps of defining a spatial coord. system locate the origin define fundamental plane choose the reference point choose signs of axes In astronomy, distance is superfluous most times two angles are enough to describe a point in space.

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**Altitude-azimuth system**

aka, Horizontal coord. system fundamental planet=horizon, reference point=north point azimuth : 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

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Equatorial System fundamental plane = celestial equator, reference point = vernal equinox right ascension = alpha = RA, 0h to 24h declination = delta = Dec, -90 to +90 degrees hour circle = great circle of constant RA, or great great circle that passes through North Pole.

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Precession Precession = rotation of the Earth spin axis (period=26,000 yrs 50 arcsec/yr) vernal equinox is marching east by 50 arcsec per year B1950 and J2000 coordinates 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!

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**Relationship among latitude, altitude, and declination**

circumpolar stars? altitude of NP = latitude of an observer Meridian : 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 object local sidereal time = RA of Meridian

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**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.

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**Galactic coordinate system**

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

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**Solar Time Sidereal time = RA of an object in transit**

Earth’s rotation rate relative to distant stars sidereal day = hours Solar time = Time tracked by the Sun (local noon is when the Sun transits) solar time = RA of the Sun + 12 hours solar day = 24 hours

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**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

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**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) days Civil calendar (Gregorian calendar) = 365 days. To compensate the difference every 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 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). 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…

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**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 – , most commonly used in astronomy (introduced by SAO to track Sputnik using 18bit number).

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**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.

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**Chapter/sections covered in this lecture : Chap 3**

In summary… Important Concepts Important Terms various coordinate systems Time system Visibility of an object for an observer zero magnitude flux great circle meridian hour circle zenith, north point transit hour angle etc. Chapter/sections covered in this lecture : Chap 3

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Roger A. Freedman • William J. Kaufmann III

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