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An article which appeared in an issue of the
Sheffield Astronomical Society Newsletter in 1994.

(Redesigned 2004 for the web, with a few corrections)



"WINTER," says the President of the A.S. during a recent meeting.   "What constelation comes to mind?"

"Orion," someone says.


Orion is indeed a very prominent constelation during winter evenings, in Northern latitudes. Yet, has it always been? And will it always be?

Some may be surprized to learn that the answer is - No. Orion will not always be a prominent feature in our winter skies. In fact, it will not always be visible at all from Britain and similar North latitudes.

In the year 14,000 AD, the familiar figure of Orion will be too far south to be seen from Sheffield ; and in 15,000 AD it will be too far south to be visible from anywhere in Mainland Britain. A similar situation existed around the year 11,000 BC.

In the year 14,000 AD Sirius, the brightest star in the whole sky, will have a southern declination of more than 62, taking it much too far south to be seen from anywhere in Britain. To compensate, however, the zero magnitude star Alpha Centauri (actually a multiple star system, the closest known system to the Sun, at a distance of about 4.3 light years), will be (for Sheffield viewers) 15 degrees above the Southern horizon at midnight around the beginning of December in the year 15,000 AD. At the moment this star is too far South to be seen from anywhere in the British Isles.

What is the reason for this change in the skies?

The answer is - Precession.

What is Precession? And what is it caused by?

The Earth is not completely spherical. It is, instead, slightly oblate. That is, it is slightly flattened at the poles, and buldges slighly at the equator, as a result of its rotation. One effect of the rotation of the Earth, and also of the fact that its radius at the poles is slightly less than at the equator, is that one appears to weigh very slighly more at the poles than one does at the equator.

Another consequence of the Earth being oblate is that the Sun and Moon exert a gravitational torque on the Earth's equatorial buldge, trying to pull it in line with the Ecliptic. Since the Earth's equator is inclined to the Ecliptic (the plane of the Earth's orbit round the Sun) by about 23, the effect of the combined gravitational torque of the Sun and Moon on the Earth's equatorial buldge is to cause the Earth's spin axis to move round, or Precess. This means that the star Polaris (Alpha Ursa Minoris) will not always be within a degree of the North celestial pole. It will travel far from the pole, before eventually starting to approach it again, returning close to its present position in about 27,800 AD.

The projection (in a northerly direction) onto the celestial sphere of the Earth's rotation axis describes a circle whose center is the pole of the ecliptic and whose radius is 23. One complete precession cycle takes about 25,800 years, after which the pole is back where it started.

In the year 2800 BC, the 3.5 magnitude star Thuban, in the constelation Draco, was the pole star. In the year 4100 AD, the 3.3 magnitude star Gamma Cephi will pass within about 2 of the pole. Further ahead, around the year 10,000 AD, the brilliant 1.3 magnitude star Deneb will be about 7 from the pole ; and around 14,000 AD the zero magnitude star Vega will pass within about 5 of the pole. Before this, around the year 13,100 AD, the 4.3 magnitude star 13 Lyrae will pass within half a degree of the pole.

Meanwhile, what is happening at the South celestial pole?

At present, the neighbourhood of the South celestial pole is rather devoid of bright stars. The South pole is marked by the 5.5 magnitude star Sigma Octans, barely visible to the naked eye. In the year 4200 AD, the 4.1 magnitude star Gamma of the constelation Chameleon will be less than 2 from the pole. In the year 5800 the 3.6 magnitude star Omega Carinae will be less than a degree from the South celestial pole. There are a number of second or third magnitude stars in this region of the sky, many of which will pass close to the pole. In the year 8100 the 2.3 magnitude star Aspidiske, again in the constelation Carina, will be the pole star, followed in 9200 by the magnitude 2.0 star Delta Velae. (By now the Milky Way is crossing the South pole, so the neighbourhood is studded with a number of moderately-bright stars, many of which take their turn at holding the honour of being Pole Star.)

Around the year 14,000 AD, while Vega is marking the North Celestial Pole, Canopus, the second brightest star in the sky, will be within 10 of the South Pole ; and around the year 22,000 AD the 0.5 magnitude star Achernar, in the constelation Eridanus, will have a South declination of about 82.

 

The Moon's orbit around the Earth is not quite in the plane of the Ecliptic, but is inclined to it by about 5. This is the reason eclipses do not occur at every New Moon or Full Moon. Eclipses occur only when the Sun and the Moon happen to pass through or close to one of the nodes (points of intersection of the two orbital planes) at the same time. This can occur at two times of year. Moreover, the plane of the Moon's orbit round the Earth is not fixed in space, but swivels round with respect to the Ecliptic in a way similar to that in which the Earth's axis of rotation precesses. This precession of the line of nodes of the Moon's orbit means that eclipses do not occur at the same times each year ; but instead the periods during which eclipses may occur (eclipse seasons) gets earlier from year to year.

The time taken for a complete revolution of the lines of nodes of the Moon's orbit is about 18 years 219 days. It is on account of this regular rotation of the lines of nodes of the Moon's orbit that eclipses occur in a fairly-regular sequence. It was known from ancient times that eclipses occured in regular predictable cycles of 18 years 11 days 8 hours. This period is known as the Saros.

 

Polaris, the 2.0 magnitude star which at present marks the North Celestial Pole, was at one time used to define the magnitude scale of brightness - until it was discovered to be slighly variable!

Its variations, however, have in recent years been observed to be petering out ; and by the turn of the century some have predicted that Polaris may no longer be variable. Thus Polaris may be approaching a significant period in its life history as a Cepheid variable.

 

Kieron Taylor.

 

P.S.

I now have my fully-registered copy of SKYGLOBE, by Mark A. Haney, the Shareware version of which is on the disks sold at the Auction held on 28 February 1994. I used this program to compile most of the data in this article. It is better and more up to date than the Shareware version, and will do things the Shareware version couldn't. Also, certain bugs in the Shareware version seem to have been corrected in my registered version.

 

Kieron Taylor.

 

1 March 1994.

(This edition 2004)

Slight modification to the HTML on the page, March 2005; page content unaltered.