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Polaris - the North Star

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But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament.
The skies are pained with unnumber'd sparks,
They are all fire and everyone doth shine,
But there's but one in all doth hold his place
 – Julius Caesar, in the play by William Shakespeare

Polaris is one of the best known stars in the sky. It's not the brightest – it only barely makes it into the top 50, appearing in some lists in 48th position1. The brightest stars are 1st magnitude. Polaris is only 2nd magnitude, so it shouldn't be a particularly noticeable star. The reason it is celebrated, however, is because of its other title: the North Star or Pole Star.

Polaris, by complete chance, happens to be almost exactly above the Earth's North Pole. As the Earth rotates on its axis and orbits around the sun, the North Pole continues to point in the same direction in space. All the other stars in the sky appear to move, but Polaris doesn't; it is at one of two points in the sky that remain stationary. The rest of the stars appear to revolve around it. This fixed point is called the Celestial North Pole.

It's one thing to look up in the night and to see Polaris. But it is strange to think that it is there all the time, day and night. It doesn't rise or set, but just sits in the same position at all times. The only reason we can't see it during the day is that the daytime sky is far brighter than the star.

Polaris isn't exactly at the Celestial North Pole; in fact it is about three quarters of a degree from it2, more than the width of the full moon, but that is close enough for practical purposes.

The other fixed point in the sky, the Celestial South Pole, is directly above the Earth's South Pole, but there isn't any bright star in that position. Southern Hemisphere sky-watchers can learn how to locate the Celestial South Pole using the patterns of the stars, but there is nothing to see.

Finding Polaris

The easiest way to find Polaris is using the pattern of stars known as the Plough (or 'Big Dipper' if you are in America). These seven stars form part of the constellation of Ursa Major, the Great Bear, and look like a saucepan3 with a curved handle to the left of it. You can see it in the diagram in the Entry about Ursa Major. The two rightmost stars are known as the pointers. Imagine a line upwards (relative to the saucepan) from these stars. This line points to Polaris. Go along this line for five times the distance between the pointers, and you will arrive at the North Star.

If Ursa Major is too low in the sky to be seen easily, it is possible to use another constellation to find Polaris. The one you want is Cassiopeia, which looks like a somewhat wonky letter W or M, depending on which way up it is in the sky.

  • If Cassiopeia looks like a W, then imagine a line joining the two extremities of the W. From the star at the left end of this line (epsilon Cassiopeiae), go 'upwards' relative to the W, at right angles to the line, for a distance twice that between the two tips of the W. You will end up very close to Polaris.

  • If, on the other hand, Cassiopeia looks like an M, then imagine a line joining the two extremities of the M. From the star at the right end of this line (epsilon Cassiopeiae), go 'downwards' relative to the M, at right angles to the line, for a distance twice that between the two tips of the M.

Using Polaris

For centuries, Polaris has been used as a navigational aid by explorers. It can tell you which direction is true north, and its height above the horizon in degrees gives you your latitude.

You can find north by imagining a line from the North Star straight down to a point on the horizon. This point is directly north of you. This is more accurate than using a compass, because the compass points to the Magnetic North Pole, which is not at the actual North Pole.

If you have a sundial, it will be most accurate if the shadow-casting part (the gnomon) is lined up parallel to the Earth's axis, and pointing to Polaris. This means that you will have to go out in the night to line up your sundial, a device that only works during the day.

Because the sky appears to rotate around the Celestial North Pole, sophisticated telescopes with an 'equatorial mount' need to be aligned to it in order to track the stars. Most such telescopes come with a chart on how to find Polaris, and how to find the actual pole from it.

Precession

Polaris is close to the Celestial North Pole at the moment, but it was not always so, and will not be so in the distant future. Although the direction the Earth's pole does not change during the day or even during the year of the Earth's orbit around the Sun, it moves slowly over centuries. It traces out a circle in the sky over the course of about 25,800 years. During this period, the Celestial North Pole will move, first even closer towards Polaris, then towards gamma Cephei, alpha Cephei, delta Cygni, Vega, tau Herculis, Thuban and finally back to Polaris again. At the time the pyramids were built in ancient Egypt, it is reckoned that the north star was the star Thuban (alpha Draconis), in the constellation of Draco.

Names of Polaris

The name Polaris is a modern one and is short for 'Stella Polaris', meaning literally 'Pole Star'. Over the years it has been known by various names such as the Greek cynosura ('dog's tail'), the Arabic Al Ruccabah or the Italian Tramontana ('beyond the mountains'). Technical designations for the star are alpha Ursae Minoris (Bayer), 1 Ursae Minoris (Flamsteed), HR424, HD8890, SAO308, and FK5-907.

There is no ancient mythology relating to Polaris. It's not a particularly notable star in itself, and its importance only arose in the last 1,000 years or so as it approached the Celestial North Pole. This was after the time when most of the mythological tales were invented about the stars.

The Star Itself

Although Polaris looks like a single star to us, telescopes reveal that there are actually three main stars and two more distant ones. A small telescope is enough to show two stars, Polaris A (the brighter) and Polaris B (less bright). These are a distance of 2,400 AU apart – that's 30 times the distance from our Sun to Neptune, the outermost planet, but only 1% of the distance from the Sun to the nearest star, Proxima Centauri. Since Polaris B is much smaller than Polaris A, it orbits around it. But the biggest telescopes can tell that what appears to be one star, Polaris A, is actually two stars, Polaris Aa and Polaris Ab, a distance of 18.5 AU apart - that's the same as the distance from the Sun to Uranus.

The star Polaris Aa is an interesting one in itself. Its apparent magnitude (the brightness as we see it from Earth) is 2, but the star is actually 430 light years away. That's a long distance – Sirius, the brightest star in the sky, is only 8 light years away. So it is clear that Polaris must in fact be very bright indeed. It is reckoned to be a giant star, with a radius 30 times that of our sun, and a mass the equivalent of five or six suns.

Some facts about Polaris (Aa):

Bayer Designation:α (alpha) Ursae Minoris
Right Ascension:2h 31m 48.7s
Declination:+89° 15' 51"
Magnitude:+2.02 (Bright Star Catalogue)
Magnitude:+1.9 (alcyone website)
Spectral Class:F7:Ib-II
Distance:430 ly
Variability Amplitude:0.27
Variability Period:3.97
Separation from Polaris B:18.4 arcseconds

In addition, Polaris is a Cepheid variable. This means that the star pulses – it gets bigger and bigger until the internal pressure is no longer able to support the size. It then suddenly collapses back down to a small size. This happens in a regular cycle every 4.0 days. In addition to the change in size, the brightness of the star changes, so we can observe this.

The Constellation

Polaris is part of the constellation of the Little Bear, which in Latin is Ursa Minor. The Little Bear looks like a rectangle (representing the body of the bear) and a curved tail. (Nobody ever explains why the bear has a tail.) Polaris is the bright star on the end of the tail. Because it is the brightest star in the constellation, it gets the designation α (alpha) Ursae Minoris.

Not the Brightest Star

You've been as constant as the Northern Star
The brightest star that shines
And it's been you, woman,
Right down the line.
 – Singer/songwriter Gerry Rafferty (1947 - 2011) demonstrates a common misconception in 'Right Down the Line'.

Astronomer Phil Plait, author of Bad Astronomy, says that it is a common misconception in the United States that Polaris is the brightest star. Why this should be is not known, but it is probably something to do with the importance of the North Star - if it's that important, it has to be bright, doesn't it? This mistake is not so prevalent in Europe, but Gerry Rafferty's song indicates that it is not completely unknown.

Get Out and Find It

If you live in the Northern Hemisphere, we recommend that the next time the night is clear and there are stars, you should go out there and find Polaris, using the instructions given above. It's an essential life skill, which you can use for aligning telescopes and sundials, impressing friends, or even finding north when you are lost in the wilderness.

1The exact ranking of stars in terms of brightness varies from list to list, as some stars vary in brightness, and some lists give double stars as two separate entries.2The declination in the year 2000 was recorded as 89° 15' 51". The Celestial North Pole is at 90°.3'Dipper' is in fact an archaic American word meaning 'saucepan'.

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