Special Sub-Topic: The Pole Star Click Here To Play: The Pole Star

For the sake of convenience, astronomers have divided the sky into 88 constellations. In which of these constellations does the current Pole Star, Polaris, lie?

Ursa Minor. Ursa Minor, or the 'Little Bear', is a relatively conspicuous constellation, visible from the latitudes 90 degrees North to 10 degrees South. It is 'circum-polar', i.e. always visible in the sky, above 20 degrees North latitude. Since the brightest stars of this constellation appear in the shape of a ladle or a saucepan, they are collectively referred to as the 'Little Dipper'. Polaris, or Alpha Ursae Minoris, is the brightest star of Ursa Minor, and is found at the tip of the 'handle' of this imaginary 'dipper'.

The great English poet William Shakespeare once wrote, "I am constant as the northern star." [Julius Caesar; Act III, Scene 1] The Pole Star, however, does not appear *exactly* fixed in the night-sky, rather, it is slightly off from the exact North direction. About how far, in degrees, is the Pole Star away from the Earth's axis of rotation?

1 degree. We assume that Polaris always points to the North- this, however, isn't true. It actually is about 1 degree away from the actual 'North' direction, as a result of which it appears to revolve around the Earth's axis of rotation making a circle about 2 degrees in diameter. Of course, there are other factors which contribute to the non-steadfast nature of the Pole Star, including the precession of equinoxes, and proper motion of stars (proper motion is the apparent change in the position of a star on the celestial sphere, which is observed due to *its own* motion, rather than that of the Earth).

Where in the sky would the Pole Star appear to an observer situated on the North Pole? (For the sake of convenience, assume that the Pole Star *does* indeed lie exactly on the Earth's axis of rotation.)

At the zenith (exactly overhead). The zenith is the point exactly overhead for any observer, and depends on the observer's location- your zenith is different from mine. The Pole Star is always at the zenith for an observer on the North Pole, that is, it is exactly overhead. All the other stars appear to revolve around it- this, however is just an apparent motion, it actually is the *Earth* which is rotating around its own axis, on which the Pole Star is located. The point exactly opposite the zenith is called the 'nadir', and as is obvious, an observer's nadir is never visible to him/her!

And where would the Pole Star appear for an observer situated on the Earth's equator? (Again, assume that the Pole star lies on the Earth's axis of rotation.)

At a fixed point on the horizon. The Pole Star always appears on the horizon, and since it lies exactly in the North direction, it is always at a fixed point, on the Northern horizon. The equator, in fact, is the only place from where you can see both the North as well as South poles of the celestial sphere at the same time. From the equator, every part of the celestial sphere is visible at some time of the year or the other, and no star is circum-polar.

Is Polaris ever visible from the South Pole?

n. The North Star is only visible from the Northern Hemisphere, and can never be seen from anywhere below the equator, let alone the South Pole! From the South Pole, only the stars in the Southern Hemisphere of the celestial sphere are visible, and these stars are ALWAYS visible- they never rise or set. The same is the case with the North Pole, from where only stars of the Northern Hemisphere can be seen.

We now have to begin to take into account yet another motion of the Earth, one which is not easily observable- the precession of the equinoxes. Think of the Earth as a spinning top: the axis of rotation of a top is never constant, the axis seems to move in a circular path (comparatively slowly), while the top keeps rotating at the same time. Similar is the case of the Earth and its axis of rotation; the axis of rotation over a certain period, goes in the form of a circle known as the 'precession circle'. Approximately how much time does the Earth take to complete one precession?

25,800 years. The precession of the Earth is primarily due to the fact that the Earth isn't exactly a sphere (it actually is an 'oblate spheroid', its diameter is more at the equator than at the poles), as a result of which gravitational tidal forces exerted by the Sun and Moon try to pull the Earth's bulge at the equator towards the plane of the Earth's orbit around the Sun. As a result of this precession, the Pole Star also changes gradually over a period of 25,800 years- the next time our Polaris will be where it is near the precession circle now, is in 25,800 years.

Because of the precession of the equinoxes, in the year 2700 BC, our good, old Polaris was not the most conveniently situated "pole star", rather, it was a rather inconspicuous star in the constellation Draco (the dragon). What is this star, also called Alpha Draconis, more commonly known as?

Thuban. Thuban is only around 1/5th as bright as Polaris is, and so is very hard to spot in areas, such as large cities, that suffer from a considerable amount of what is called 'light pollution'. It was the naked-eye star closest to the North Pole of the Celestial Sphere from about 3000 BC to 1900 BC, it being the closest to the North Pole around the year 2700 BC.

In about 14,000 AD, an extremely bright star in the constellation Lyra is going to become the Pole Star, and will by far be the brightest Pole Star of all. What is this star, also referred to as 'Alpha Lyrae', better known as?

Vega. Vega is the 5th-brightest star in the night-sky, after Sirius, Canopus, Alpha Centauri, and Arcturus. It is also relatively very close to the Sun, only 25 light-years away. Vega, along with two other stars- Altair and Deneb- forms the so-called "Summer Triangle", as these three stars are very clearly visible in the summer-sky of the Northern Hemisphere. (By the way, the three incorrect options in this question, Canopus, Achernar and Fomalhaut, are all stars in the Southern Hemisphere, and so the question of any of them becoming the Pole Star never arises!)

Now having the Pole Star around isn't of much help if you're no good at locating it. The constellation in which it is located is not all that conspicuous, and spotting it can prove to be a real head-ache if you aren't familiar with the night-sky, and from a big city, most of the stars of the constellation are completely invisible. And so we look to our easiest method of finding Polaris- neighbouring constellations, the most prominent of which is Ursa Major, or the Great Bear. In Northern latitudes, this constellation is circum-polar, i.e. it is always visible. The brightest stars of this constellation are in the shape of what appears to be a sauce-pan or a ladle (the type you use for serving soup or juice at parties), and so they are collectively called the "Big Dipper". If the line joining 2 of the stars of Ursa Major is extended, you invariably hit the Pole Star, thus these 2 are very aptly referred to as the 'Pointers'. What are the names of these 2 stars?

Dubhe and Merak. Another constellation with which the Pole Star can be easily located is Cassiopeia. Cassiopeia is a constellation shaped like the letter 'M', just bisect the larger angle of the 'M' and you're there at Polaris! Since Ursa Major and Cassiopeia are nearly diametrically opposite, in the Northern Hemisphere, either one of the 2 constellations is visible at any given time.

Well, by now, I'm sure most of our Southern Hemisphere players have begun muttering curses at the quiz-maker and complaining about how Northern Hemisphere-centric this quiz is, and so, to make amends, I've included one question about the Southern Hemisphere... Like the Pole Star in the North, there is, as well, a Pole Star in the South, except...erm...it isn't nearly as visible. Having an apparent magnitude of around +5.5 means that it can be just seen with the naked eye, only with a lot of effort, and in big cities is entirely invisible. What is the name of this star, the closest visible star to the South Pole of the celestial sphere?

Sigma Octantis. Sigma Octantis (also called 'Polaris Australis') is found in the constellation Octans, and like I said, is just barely visible to the naked eye; hence, people don't make as much of a deal about it as they do Polaris! The easiest way to locate the South celestial pole is with the help of the Southern Cross (or Crux), a constellation in the shape of a cross. Just extend the major axis of the cross around 4.5 times its length and you're there! Note that Sigma Octantis is only visible from the Southern Hemisphere and Crux only from South of around 20 degrees North. === I hope everyone, dwellers of the Northern and Southern hemispheres alike, enjoyed playing this quiz!