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48 AUTOSTAR CCD PHOTOMETRY

Similarly, a star situated at the South Celestial Pole (SCP) would

have δ = –90°, while those between that pole and the celestial

equator have declinations of –90° ≤ δ ≤ 0°.

NCP

Earth

Equ

+90

-90

+30

-30

Celes

rial

SCP

Figure B-2. Illustration of a Star's Declination.

Determining a Star's Hour Angle (HA)

As illustrated in Figure B-3, The observer's celestial meridian is

the north-south line passing directly overhead (i.e., through the

Zenith). The hour angle, HA, of a star is the amount of time since

the star crossed the celestial meridian or until the crossing will

occur. Stars East of the meridian are designated either as a

negative value or with the symbol E, while stars to the West have

positive values or a symbol W. The HA of a star increases with

time as the celestial sphere rotates.

Mathematically, the Hour Angle is defined as:

HA = (LST – α) hours

Note: HA is defined in hours, but must be converted to degrees

(angle). To produce this, multiply HA in hours by 15 (the sky

appears to rotate 15 degrees per hour).

HA = (LST – α) * 15 degrees

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