iOptron 6001 Understanding Celestial Motion and Coordinates, Understanding celestial coordinates

Celestial equator

2.Understanding Celestial Motion and Coordinates

Understanding where to locate celestial objects and how these objects move through the sky is fundamental to fully appreciating astronomy as a hobby. Most amateur astronomers use the same visual path (or star-hopping) method for locating celestial objects. To do this they use maps of the sky or an astronomy program that identifies bright stars and constellations of stars that serve as "roadmaps" and "markers" in the sky. These visual reference points guide amateur astronomers in their search for astronomic objects and although the visual path method is the preferred approach—giving thought to whether or not to use circles of digital coordinates for locating objects is desirable as your telescope offers this function. Be warned however, when compared with a visual path approach, looking for objects using circles of digital coordinates requires a greater investment in terms of patience and time in order to achieve a more precise alignment of the telescope's polar axis on the celestial pole. This is partially why the visual path approach is preferred since it’s the simplest (and quickest) way to get started.

Celestial North Pole (close to the North Star (Polaris))

Earth's rotation

Right ascension

Celestial South Pole

Figure 2: Celestial Sphere

Understanding how astronomic objects move

Given the earth's rotation, celestial bodies (stars) appear to move from east to west along a curved trajectory across the sky. The trajectory that they follow is known as the right ascension line (R.A.). The angle of the trajectory that they follow is known as the declination line (Dec.). The right ascension and the declination form a system that is similar to

the terrestrial system of latitude and longitude.

In the system of R.A. and Dec. coordinates, stars are projected onto the "celestial sphere", i.e. onto the imaginary sphere where all of the stars appear to be located.

Understanding celestial coordinates

In the system of celestial coordinates, the poles are defined as being the two points where the earth's rotation axis, when prolonged infinitely to the north and to the south, intersects with the celestial sphere. Consequently, the celestial North Pole is the point in the sky where the prolongation of the earth's axis passing through the North Pole intersects the celestial sphere. In fact this point in the sky is located close to the North Star or pole star (Polaris).

So-called "longitude lines" are drawn on the earth's surface between the north and south poles. In the same way, "latitude lines" are drawn along an east-west direction, parallel to the earth's equator. The celestial equator is simply a projection of the earth's equator into the celestial sphere. Just like on the earth's surface, imaginary lines have been

drawn on the celestial

		sphere to form a grid of
		coordinates.		The
Star
		positions of	the	stars
		on the earth's surface
	Declination	are specified	by	their
		latitude and longitude.

The celestial equivalent to terrestrial latitude is called "Declination" or simply "Dec", expressed in degrees, minutes, and seconds north (“+”) or south ("-") of the

celestial equator.

Consequently any point located along the celestial equator (e.g.

passing through the constellations of Orion, Virgo and Aquarius) is specified by its declination of 0º0’0”. The declination of the North Star or pole star located very close to the North Celestial Pole is +89.2º.

The celestial equivalent to terrestrial longitude is called "Right Ascension" or "R.A.", expressed in hours, minutes and seconds from a "zero" R.A. line defined arbitrarily and which passes through the

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