Can We Observe x tonight

1
Can We Observe x tonight?

• Where is it?
• When is it?
• How bright is it?

2
Coordinates need three things

• A scale
• An origin
• A direction

3
Latitude and Longitude

• Scale degrees (decimal vs. sexigecimal)
• Origin Equator and Greenwich
• Direction North/East are positive

How do you know your latitude and longitude?
4
Altitude and Azimuth
Sky is inside of globe
Problem Its too local
5
Right Ascension and Declination
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Circumpolar
7
Hour Angle
MOT Hour angle of Star
MOV RA of Meridian
Pole
TOV RA of Star
eridian
tar
HA MOV - TOV
bserver

• rising
• setting

ernal Equinox
8
Ecliptic
9
Galactic
10
Sidereal vs. Solar Time
11
Equation of Time
Mean Solar Time - Apparent Solar Time
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(some) Time Systems

• Sidereal Time (stars)
• Apparent Solar Time (sundial)
• Mean Solar Time (average sundial)
• Local Civil/Mean Time (average sundial here)
• Standard Time (average sundial zones)
• Universal Time/Greenwich Mean Time (average
  sundial in Greenwich, UK)
• Greenwich Sidereal Time (stars in Greenwich)

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Some Relationships

• Eastern Standard Time (EST) UT - 5h
• Longitude (GMT - LMT) x 15 deg/h
• GMST HA of VE at Greenwich
• LMST RA of stars at your meridian
• HA LMST - RA
• Sin(Alt) sin(dec)sin(lat)
• cos(dec)cos(lat)cos(HA)

14
Julian Date

• Number of days since noon, Jan 1, 4713 BC.
• JD 2451544.5365(year-2000) DoYLeap Days
• GMST 6.697374558 0.06570982441908(JD-2451
  545.0)
• Need to subtract off 24 until GMST lt 24.

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How To Get Altitude From RA

• Get UT now
• Get hours from last midnight
• Get JD0 of last UT midnight
• Multiply 2 by 1.00273791 to get Sidereal
• Get GMST0 from 3
• Add 4 and 5 to get GMST
• Add longitude/15 to get LMST
• HA LMST - RA
• Use trig formula to get Altitude

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Does Your Head Hurt Yet?
Just wait...
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Precession
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Precessionof theEquinoxes