Thousands of years ago, ancient Greek astronomers calculated that the track of the Earth's axis is constantly, even if in a very slow way, shifting in a uniform pattern. The change is very similar to the manner a spinning top slowly leans one way and then another as it slows down. It is a wobble that occurs as its axis changes direction.
This odd movement of the planet is due to several factors, the most important of which is something called 'precession'. Precession stems from the fact that the Earth is not a perfect sphere. It is in fact about twenty-seven miles longer around the Equator that it is around the Poles. The Earth then is oblate, or fat around the middle like middle-aged spread, but it is due to the spinning not to its age.
If you imagine the Earth with its Poles off centre. Then rotate that image and you will find that any point, except the exact centre of the axis, will move in a circle. But very, very slowly. So slowly that it takes 26,000 years to go full circle and get back to where it started from.
This point then, any point you choose, is very slowly changing its location in relation to the stars because the axis is rotating too. The result of this is that, what we call the North Star (formally known as Polaris, which is actually one degree off true North) will not be above our North Pole one day. In fact, by about 15,000 AD, Vega will be almost over the North Pole, although it will be about four degrees off true North. But even this will not endure, and by 28,000 AD, Polaris will be back above where it is nowadays.
One of the effects of the precession is that seasons change. They modify the dates that they take place, so that Summer could come earlier or later. The amazing thing about our calendar is that it is corrected for that (with the leap year). If it were not, the vernal or Spring equinox would move over 13,000 years from March 21st to September 21st., which is the date of the autumnal equinox, precisely half a year later.
It is for this reason that the precession of the Earth is generally referred to as the "Precession of the Equinoxes". Although the precession of the equinoxes is very lengthy, it can be readily observed. The correct year of 325.25 days is the length of time from one vernal equinox to the next vernal equinox, however, it takes 20 minutes and 24 seconds longer for the Sun to appear in exactly the same place in relation to the stars behind it over the same period, which is why accurate star maps have to be stamped with the exact time and date to which they relate.
This odd movement of the planet is due to several factors, the most important of which is something called 'precession'. Precession stems from the fact that the Earth is not a perfect sphere. It is in fact about twenty-seven miles longer around the Equator that it is around the Poles. The Earth then is oblate, or fat around the middle like middle-aged spread, but it is due to the spinning not to its age.
If you imagine the Earth with its Poles off centre. Then rotate that image and you will find that any point, except the exact centre of the axis, will move in a circle. But very, very slowly. So slowly that it takes 26,000 years to go full circle and get back to where it started from.
This point then, any point you choose, is very slowly changing its location in relation to the stars because the axis is rotating too. The result of this is that, what we call the North Star (formally known as Polaris, which is actually one degree off true North) will not be above our North Pole one day. In fact, by about 15,000 AD, Vega will be almost over the North Pole, although it will be about four degrees off true North. But even this will not endure, and by 28,000 AD, Polaris will be back above where it is nowadays.
One of the effects of the precession is that seasons change. They modify the dates that they take place, so that Summer could come earlier or later. The amazing thing about our calendar is that it is corrected for that (with the leap year). If it were not, the vernal or Spring equinox would move over 13,000 years from March 21st to September 21st., which is the date of the autumnal equinox, precisely half a year later.
It is for this reason that the precession of the Earth is generally referred to as the "Precession of the Equinoxes". Although the precession of the equinoxes is very lengthy, it can be readily observed. The correct year of 325.25 days is the length of time from one vernal equinox to the next vernal equinox, however, it takes 20 minutes and 24 seconds longer for the Sun to appear in exactly the same place in relation to the stars behind it over the same period, which is why accurate star maps have to be stamped with the exact time and date to which they relate.
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