8/9/2019 Positional Astronomy 4 Sidereal Time

1/1

1.4 Sidereal Time

The circumpolar region: There is a whole region around the pole for each observer of stars that as

their celestial sphere rotates (or the earth rotates) never set below the horizon.

The circumpolar star: A circumpolar star is a star that, as viewed from a given latitude on Earth, never

sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles.

Circumpolar stars are therefore visible from said location toward nearest pole for the entire night on

every night of the year (and would be continuously visible throughout the day too, were they are not

overwhelmed by the Sun's glare).

Which of the following options best describes the amount of the visible sky covered by the circumpolar

region for an observer looking out from the North (or South) Pole? Answer: All of the sky. Why?

Because it is asking the amount of the visible sky, so how much of the sky we see in the poles is a

circumpolar region? Everything, because the whole sky above us just rotates in the poles, it never sets

below the horizon.

What we can see

- As sky rotates about the celestial pole near North (or South) pole never set (circumpolar).

- Stars near South (North) celestial pole never visible (they are at the other side of the Earth).

- Stars near celestial equator rise, move to the West across the sky, and set.

http://astro.unl.edu/naap/motion2/animations/ce_hc.html Zenith

- Is the point with: Declination = Latitude and Right Ascension = Sidereal Time

Sidereal time is the celestial meridian coinciding with the local meridian (the one that includes ourzenith).

[Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their

telescopes to view a given star in the night sky. Briefly, sidereal time is a "time scale that is based on

the Earth's rate of rotation measured relative to the fixed stars."] Changes with time: 24 sidereal

hours = One full rotation of the Earth. So we can use stars to measure time! In one (sidereal) hour,

the Celestial Sphere shifts by one hour of RA. Changes of longitude at 1h/15. "Right ascension

(abbreviated RA; symbol ) is the angular distance measured eastward along the celestial equator

from the vernal equinox to the hour circle of the point in question.[1] When combined with

declination, these astronomical coordinates specify the direction of a point on the celestial sphere in

the equatorial coordinate system."

Summary:

Finding a Star

- Star is highest at meridian crossing when sidereal time is its Right Ascension.

- At this time, the Zenith Angle is |Declination-Latitude|.

- Altitude is 90 - Zenith Angle.

- Azimuth is 0 if Declination > Latitude.

- Azimuth is 180 if Declination < Latitude.

- To find a star earlier/later, rotate East/West by 15/h.

-

We need to know how to tell sidereal time.