Polaris' Changing Declination at Upper Culmination

Polaris's declination change (mainly due to precession) is so small on a day-to-day basis that it's probably not measurable with a transit instrument. Are you sure it is at upper culmination each time you measure it? If you are measuring it at the same time (civil time, e.g., at 10:00 PM) every day then it is not at upper culmination each time.

Due to the Earth's orbit around the Sun, the stars cross the meridian roughly 4 minutes earlier from night to night. In June, Polaris is more-or-less 'below' the pole at 10 PM. In September, Polaris is 'to the right' of the pole at 10 PM. In December Polaris is roughly 'above' the pole around 10 PM, and so forth. So if Polaris is at upper culmination at 10:00 PM on one day, then it would be at upper culmination at about 9:56 PM the next day.

There are also corrections that need to be applied due to the Earth's orbit being elliptical. When the Earth is near perihelion (in January) the Earth is moving faster in its orbit, whereas it moves slower near aphelion (in July). This is what generates the 'analemna' you often see associated with sun dials and the phrases "sun fast' and 'sun slow'.

Hello:

Yes, I could clearly see that Polaris was going upwards in the cross hairs of my theodolite and then there was a period of about 15 minutes when it seemed to only move westwards, then after awhile it began to descend. This was as you say, around 0930PM and so I concluded that the upper culmination had ocurred. Using my Longitude correction for Universal time the value in the ephemeris is the same as what I got. My instrument read 19degrees 10 minutes and when I calculated my latitude it came out very close to exactly where I am. When I look in the 2010 Sun and Polaris Ephemerides I see the listing for time of Upper Culmination as well as the Declination which when subtracted from 90 will give you the angle away from true north. But as I said earlier the thing that confuses me is the declination at Upper culmination.

If the distance from theoretical true north to Polaris is a fixed value (for our period at least) then the declination should always be the same at upper culmination. But I must be wrong because if you read that ephemeris, which must be right, you will find that the declination is not a constant at culmination. It changes, albeit only a small amount, but it changes. And if this is true how can I get an exact value for my Latitude using Polaris.

I hope you understand what I'm saying and since I'm sure I'm wrong what I need is an explanation of what mistake I'm making.

I'm going to look up the exact figures I got and send them to you.

Thanks

John

There is also an annual aberration due to the Earth's motion, as described in the article below. This aberration is probably measurable with your theodolite, so it's another small correction you would apply to get a truly precise measurement.

http://en.wikipedia.org/wiki/Annual_aberration

Thanks, this is probably what I needed to know. I'll look it up and keep you posted.

Regards

John

Over the course of time, the North Star changes. I believe every 20,000 years, I'll have to google that....

But, right now Polaris is within one degree of true north, but at other times the North Star has been and will again be Thuban (the brightest star in the constellation Draco), Vega (the brightest star in the constellation Lyra), and Alpha Cephei (the brightest star in the constellation Cepheus).

The North Star changes over time because the direction of the earth's axis changes slowly over time. Since by definition the North Star is the star most closely aligned with the earth's axis, as the axis moves the nearest star changes too.

I did know this, but not the particulars, so I did have to google this

This could be totally wrong and a load of crap but what about the effect the Moon has on the Earth.

In reality the Moon and Earth orbit each other around a central point called the barycentre which is basically the centre of gravity of the combined masses. For the Earth and Moon this lies somewhere below the surface of the Earth but quiet a distance from its centre. The end result is that the two bodies seem to rotate around the barycentre rather than the centre of the Earth.

Could not this rotation around the barycentre be causing the variation you are seeing?

This is over my head. So I shall not comment on the technicalities.

I do look at the stars and consolations, when I am far enough from the light pollution of the major population centers. I also try to read as much as I can.

One of the best events, other eclipses, was watching the Auroral Borealis while lying down on a beach on an island, at a latitude that you should not be able to view it from.

Yes the beer did enhance the moment

I just wish to brush up against those who have knowledge on this subject.

I just want to learn.