# Astronomy and Space SciencePage 4

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A: The IAU divides the celestial sphere into 88 constellations (regions) with precise boundaries (yellow dashed lines in the figure).

More:

Each star belongs to exactly one constellation.

The term “constellation” is also less formally used to describe a group of star visibly related to each other in a pattern, such as those connected by green lines in the figure. However, in such a scheme, some stars such as Sirrah in Andromeda, may be considered as both the head of Andromeda or part of the Square of Pegasus. Also, stars not connected by patterns still need to be assigned a constellation.

Slide 25

## How does the coordinate systems on the Celestial sphere look like?

A: As shown on the graph: the longitude and latitudes of the Celestial sphere are called RA (right ascension) and DEC (declination). DEC runs from +90° to -90°. RA runs from 0 to 24 hours. Each hour has 60 minutes, and each minute has 60 seconds, just like the clock. The RA of zenith of a fixed location increases by roughly 1 hour for every hour in time.

(Note: Do not confuse the minute with arc minute which is

1/60°, both measure angles.)

Refer to the previous figure, the light blue lines are RA and DEC lines.

Slide 26

## Where exactly is the center of the celestial sphere?

A: The center of the celestial sphere is the observer. In other words, each observer has a celestial sphere.

More:

The celestial sphere is a device used to represent the direction of celestial objects for observation. For example, someone in Beijing would see the Moon’s position a little differently from someone in Hong Kong, due to parallax of the observing locations. Therefore, it only make sense to have a different celestial sphere (and the objects on them) for each for observer. Another example is the satellite or space station, which, due to there close distance from Earth, depends greatly on the location of the observer. Also, if one were to observe from Mars, it would not make sense if the celestial sphere is centered on Earth!

Note however that in most situations, we are observing on the Earth and most objects are far away so it is convenient to set the center of the Earth as the center of the celestial sphere.

Slide 27

## I heard that the definition of the ecliptic plane has been changed, is it?

A very first definition is the ecliptic plane is the plane in which the Earth orbits.

A few amendments have been made since then.

In 2006, the IAU adopted a new definition:

the ecliptic pole is explicitly defined by the mean orbital angular momentum vector of the Earth-Moon barycenter in an inertial reference frame.

This change is to better agree with dynamical theories, however, the actual change in value is extremely small.

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