Observing the Sky

NOTE: It will not be our usual practice to post questions from class here on the website; it is your responsibility to write them down during class (as you do for lecture notes). This set is posted to give you a feeling for the level of depth and detail that you should be able to achieve in answering them.

What is the celestial sphere?

It is an imaginary sphere around the Earth to which the stars appear to be attached.  It seems to rotate around us from East to West with a period of 24 hours, thus causing things like the Sun and Moon to "rise" and "set"

What is a constellation?

It is an area of the celestial sphere denoting a direction in space away from the Solar System.  Constellations are marked by fanciful "connect the dots" imaginary pictures (such as the Big Bear or the hunter Orion), but those pictures have no real, physical significance.

How many constellations are there and how were they named?

In the modern, "official" list of constellations, drawn up by the International Astronomical Union in the 1930's, there are 88 constellations.  Their names come mostly from ancient mythology.

What's the difference between a “2nd magnitude star” and a “5th magnitude star”?

"Magnitude" is a quantity that expresses stars' brightnesses, and the lower the number, the brighter the star. (The fainter the star, the larger its numerical magnitude is.) Thus, a "2nd magnitude star" is brighter than a "5th magnitude star".

What is the difference between the celestial equator and the ecliptic?

See Chapter 2 and the glossary of your textbook for definitions of these terms. The celestial equator and the ecliptic are tilted by 23 1/2 degrees relative to each other because the Earth's spin axis is tilted by that amount relative to a perpendicular to its orbit plane.

What is precession? How long does it take to complete one full cycle?

While the tilt of Earth's axis relative to its orbit plane around the Sun remains constant over time, the direction that the axis points slowly twists around due to the Moon's and Sun's gravitational tugs on Earth's equatorial bulge. That slow change in the orientation of the axis is called precession. It takes 26,000 years to complete one full cycle.