Given that you are living in the northern hemisphere
of the planet like me, you have probably noticed one particular star, which
happen to be pointing towards the north, always remaining stationary no matter
the season or what time at night you look at it. This star is Polaris, the
North Star. Why does this star always point towards north? Why does it appear to be
fixed in the sky?
The answer is fairly simple actually. Polaris is
positioned in such a way that the line of Earth’s axis of rotation projected to
the space is very close to it. As it is the only bright(ish) star in the vicinity of
the line of rotation of earth, it appears to be fixed in the sky while the
other stars appear to be moving in the opposite direction of rotation of the
Earth.
What Happens When You Go Towards The South?
If you were to look up at the sky at the North Pole, you’ll
see Polaris directly overhead while the other stars move around it throughout
the night. As you start to move away from the Pole, towards south, it still is
the center of the movement of the stars while it remains stationary, however, it
appears further down towards north. You cannot view it in the southern
hemisphere as you can probably guess. In fact, there is no such star in the
southern hemisphere as no bright star is aligned with the axis of rotation of
Earth.
Is Polaris Truly Fixed/ Stationary?
It appears that the star remains fixed in the sky. If
you were to watch it all night, however, you may or may not observe a movement.
If you set a camera and if you were to make a time-lapse video, you’ll see it
makes a little circle. This is not significant enough though. This is because the star is
not perfectly aligned with our axis with a difference of about three quarters
of a degree. This is the reason it makes its own little circle.
Will Polaris Always Be Our North Star?
The Earth ‘wobbles’ in regular intervals. This
phenomenon is called precession. So what kind of motion is this? Imagine a
spinning top. When you gently touch it or it is slowing down, the tilted axis
of rotation gyrates in circular motion which traces a cone. What causes the
earth to wobble? The gravitational pull of the Sun and the Moon. In addition to
this, the shape of earth makes it easier. Earth has an equatorial bulge and
this bulge gets the gravitational tug and makes the Earth wobble.
Precession makes the Earth to trace out an imaginary
circle every 26,000 years. You can see clearly that no star can always stay
aligned with the axis of the Earth.
When the Pyramids were being built few thousand years
ago in ancient Egypt, our north star was Thuban in the constellation Draco the
Dragon. The current North Star is about 433 light years away from us and is a
part of the constellation Ursa Minor. In 13,000 years, blue-white star Vega,
the brightest star in the constellation of Lyra the harp will be close to North
Celestial pole. However, it will never be as close as Polaris is.
More About Polaris
Polaris is the brightest star in the constellation
Ursa Minor. The seven stars from which a bear is derived in this constellation
are also called as Little Dipper. Polaris lies at the end of the handle of the
Little Dipper.
Polaris is approximately 433 light years further away
from us. It is a triple star system with Polaris Aa being the primary star in
orbit around its smaller partner Polaris Ab. This pair is in orbit around
Polaris B. Polaris B was discovered by the famous astronomer William Herschel
in August 1779 using a reflecting telescope. Polaris’ luminosity is about 4000
times our Sun.
It has regular cycles of brightening and dimming. This
kind of star is called Cepheid variable. In the past few decades, Polaris is
getting brighter for reasons that are unknown still. In fact, scientists say it
could be about 4.6 times brighter today than it was in ancient times.
CONCLUSION
Polaris is aligned with the axis of rotation of Earth.
This is why the position of the star appears to be fixed. Despite this, due to
not being aligned perfectly, it still moves in a little circular motion in the
night sky. Polaris, however, won’t be our north star forever due to Earth’s
precession. The axis would not be aligned with the star. The other stars which
would be close to the axis of the earth will not be as close as Polaris
is.
Comments
Post a Comment