Introductory Astronomy
Week 1 – Posi5onal Astronomy Clip 4 – Sidereal Time
1
What We Can See • As sky rotates about celes5al pole stars near North (South) pole never set (circumpolar)
• Stars near South (North) celes5al pole never visible
• Stars near celes5al equator rise, move West across sky, and set
2
3
Sidereal Time • Zenith at Decl = La5tude RA = Sidereal Time
• Sidereal Time is celes5al meridian coinciding with local meridian
• Changes with 5me: 24 sidereal hours = One full rota5on of Earth
• Can use stars to measure 5me! In one (sidereal) hour Celes5al sphere shiSs by one hour of RA
• Changes with longitude at 1h/15° 4
Summary: Finding A Star • Star is highest at meridian crossing when sidereal 5me is its RA
• At this 5me Zenith angle is |Decl-‐La5tude| • Al5tude is 90° -‐ Zenith angle • Azimuth is 0° if Decl > La5tude 180° if Decl < La5tude
• To find star earlier/later, rotate East/West by 15°/h • Need to know how to tell sidereal 5me
5
Credits • Sky Simula5on: Starry Night hap://www.starrynight.com/
• Astronomy Anima5ons: University of Nebraska-‐Lincoln Astronomy Educa5on Group hap://astro.unl.edu/
• Star Trail Photos: Australian Astronomical Observatory hap://www.aao.gov.au/images/
6
Top Related