3.2 : Observing the Night Sky

The lines in a grid system used to locate objects in the night sky are called...
Right ascension (RA) and declination (dec).
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What is the celestial sphere?
An imaginary sphere that the Earth is the centre of.
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How is declination expressed?
As angles (positive north of the celestial equator and negative south of the celestial equator).
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Zero right ascension is the point at which...
The Sun moves from the southern to the northern hemisphere.
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How is right ascension expressed?
In hours and minutes (where 1 hour is equivalent to 15 degrees) eastwards.
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What is the Zodiacal Band?
An area of sky straddling the ecliptic by about 8° on either side that contains the constellations of the zodiac and is the region in which the planets and the Moon are located.
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Polaris, the North Star, has a declination of...
+90°.
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Why do the stars appear to rotate in an anticlockwise sense about Polaris when looking north?
Because of the Earth's rotation from west to east.
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What makes a star circumpolar?
A star is circumpolar if it does not dip below the northern horizon during the course of a night.
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A star will be circumpolar from a given latitude on the Earth provided that its declination is...
Greater than 90 - latitude.
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How can circumpolar stars be used to determine the Earth's rotation period?
By producing long-exposure photography of star trails produced by the stars. The camera is pointed at Polaris for exposure times of more than 2 hours to give arcs large enough to be measured. Rotation period = (exposure time)(360°)/(mean arc angle).
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What is the apparent motion of the stars from the southern hemisphere?
They appear to rise in the east and set in the west.
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What happens when stars culminate?
They cross the observer's meridian -- at this point, they are due south.
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At what position do stars appear their brightest and why?
At their highest elevation in the sky because at this point their light has the least amount of atmosphere to pass through, so less light is absorbed.
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How can the time at which a given star culminates be predicted?
By using star charts and computer software.
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Why won't a given star culminate at the exact same time each day?
Because it takes only 23h 56m for a star to return to an observer's meridian, not 24h.
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What are some points to consider when planning a suitable date for an observation session?
The Moon's phase (should ideally be new); the weather forecast (clear nights are ideal); the likelihood of a meteor shower (perfect for naked-eye observing); the visibility of a comet, planets or interesting Messier objects.
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What is some equipment you are likely to need for naked-eye observations?
A comfortable reclining chair with a small table; a torch with a red filter; a star chart with a pencil and a rubber; warm clothes and refreshments.
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What are the two types of cell in the eye's retina and what differentiates them?
Cones (colour-sensitive) and rods (not colour-sensitive).
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For any serious observations to be carried out, the eye must become...
Dark-adapted (the retina's rods become fully sensitive to light).
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Why is averted vision used to observe some bodies?
If a star or nebula is not bright enough to stimulate the retina's cones, it will not be seen if looked at directly (such as the Andromeda Galaxy).
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What is the Messier Catalogue?
A list of 'fuzzy' objects, many of which are just visible to the naked eye. It was completes by French astronomer Charles Messier in 1781
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Card 2

Front

What is the celestial sphere?

Back

An imaginary sphere that the Earth is the centre of.

Card 3

Front

How is declination expressed?

Back

Preview of the front of card 3

Card 4

Front

Zero right ascension is the point at which...

Back

Preview of the front of card 4

Card 5

Front

How is right ascension expressed?

Back

Preview of the front of card 5
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