#star motion

[autodidact-adventures]

When a star has rotated around the fixed point - or the axis - once, that means the earth has completed one spin on its axis.  It’s called a sidereal day.  But!!  It is not a full day by the clock!

A full day by the clock is when the sun has transited twice.  Remember that the sun is a star, too - and goes in circles just like the other stars.  It’s just closer to us, that’s all.

At the north pole, the fixed point/axis is right above you, and all stars are circumpolar around it.  This includes the sun.  If the earth was straight up-and-down, the sun would be a full 90° away from the axis.  But the earth is tilted, so when the north pole has summer (when that pole is tilting towards the sun), the sun is just above the horizon.  It’s circumpolar, like all the others, so it never sets - so you have constant daylight as it moves in a very large circle around your horizon.  And for winter, vice versa.

People don’t live bang smack on the north pole, though.  Just like in the last post, the further south you go, the more the fixed point tilts forward, and the stars with it.  The sun is only circumpolar for a while.  As its circle tilts, it drops below the horizon - but only for a short time, you can still see most of the circle - and the more it tilts as you move south, the more of the circle happens below the horizon.  Eventually, you get to the equator, and half of the circle is below the horizon.

So the sun transits just like a star.  You’ll always see its upper transit - when the sun is highest in the sky - but not its lower transit, unless you live very near one of the poles.  A full transit of the sun is a solar transit - but it’s slightly longer than a sidereal transit.

The earth has rotated once on its axis - but it has also moved a bit more around the sun.  So after a full rotation, the sun isn’t quite in the same place  The earth has to rotate a bit more for the sun to complete its transit.  As it’s moved about 1° around the sun, it has to rotate another 1° to compensate - 1/360 of a day, or 4 minutes.  So a full day is 1 & 1/360 of a rotation. 

[autodidact-adventures]

If you stand outside on a clear night, you will see half of the celestial sphere above you.  Surrounding you is the circle of your horizon - it is a plane and you are in the centre of it.  Directly above you is your zenith; directly below you is your nadir.

Turn to face directly north.  Imagine a semicircle arcing above you, drawn south to north, passing through your zenith, with you in the centre facing the north end.  This is your meridian.  A line drawn between the points where it meets the horizon would go north-south; a line perpendicular to that east-west.  Everyone on the same longitude has the same meridian.  The prime meridian at 0° is at Greenwich in London.

If you were at the north pole, the stars would appear to be rotating in concentric circles around a fixed point right above you.  This is where the earth’s axis is pointing: the earth rotates around this axis, and so the stars appear to rotate around it, too (in the other direction.)  These stars, while you’re at the north pole, are circumpolar - they do not sink below the horizon.

If you moved downwards into Russia (for example), you would no longer be at the centre of everything.  Facing north, the north pole is forwards.  So the fixed point that the stars rotate around has “moved” forwards, too.  It’ll be at an angle in front of you - although still quite high up.  And now, because the fixed point has moved downwards, all the stars have, too - and you can’t see the bottom of the largest circles anymore.  Those stars are completing their circling below your horizon.  They are no longer circumpolar - most of the stars still are, though.

Now of course you’re not going to see the full circle - because half of it takes place in daylight!  What I mean, though, is that it takes place within your area of vision.  (Ignore night and day for the moment.)

The further you move towards the equator, the more that fixed point moves downward, and the fewer stars are circumpolar.  Eventually, you’ll get to the equator.  You’ve moved 90° around the earth - and so the fixed point has gone down 90° from right above you.  That means it’s now right on the horizon.  You can’t see below your horizon - that means that half of each circle is out of sight.  None of the stars are circumpolar at all.

When a star reaches its highest point in its circle, this is its upper transit.  At its lowest point, it’s the lower transit.  You can see all of the upper transits - but not all of the lower transits, unless you’re right at the pole.  When the star has completed two transits, it’s rotated once around the axis (or rather, the earth has).  This takes about a day.

After its upper transit, the star culminates.  The celestial equator surrounds the earth's equator, just further out.

Any change in a star's position relative to its neighbours is its proper motion. The star that shows the most proper motion is Barnard's Star, but it's less than 1/3° per century.