Sidereal RA?

[Stub Mandrel]

The RA dial of my mount reads 24 hours 00 minutes.

But a sidereal day is 23 Hours 56 minutes and a few seconds.

If the RA of a star is when it crosses the meridian, surely none of them can have an RA greater than this lower figure?

Am I missing something?

<edit> just played with stellarium. I am missing something - this is why the sky moves 4 minutes along every day, isn't it... so a 4 minutes worth of stars cross the meridian twice each day.

[ronin]

σ Cas is defined as an RA of 23 hrs 59 minutes 00.53 seconds so RA can exceed 23 Hours 56 minutes + whatever.

The presumption seems to be that a sidereal day is not applicable, or something really weird is done.

Suppose they could simply split a Sidereal Day into 24 units and just call them hours, so still 24 hours just 24 slightly different hours.

[Gazabone]

Good question, but I think you're mixing up two slightly different things; a system for giving coordinates across the sky and how quickly they process across the sky. The celestial coordinates would still work (as far as I understand them) if instead of hours, minutes and seconds of RA, we had them marked out in (say) 1000 equal intervals, or just in degrees like declination, after all, it works for altitude azimuth coordinates and a sidereal day still lasts 23 hours, 57 minutes and 4 seconds approximately.

[Scooot]

It still takes 24 sidereal hours for the earth to spin 360° but because the earth moves around the sun 1° in a day, the sun is in the same position after only 23 hours 56 minutes.

[ronin]

The sidereal rate is 1 rotation of the earth relative to the stars, the solar one is 1 degree more (about 4 minutes).

It is because we measure a solar hour/day as when the sun is back at the same position - usually overhead - and as we orbit the sun this take about 4 minutes more every rotation.

So a solar day is fractionally longer then a sidereal day.

So it seems the scale is in sidereal hours, or 24 units of one rotation relative to the stars not one rotation relative to the sun.

It is the close proximity of the sun that is really the hiccup, but it could be worse we have 365 days a year and fortunately that is close to the 360 degree we have in a circle.

[Stub Mandrel]

I've spent an hour timing revolutions of my RA gear, and as far as I can see its pretty accurate but the mind wanders badly doing something like that (you start trying to see if you can see the RA dial move - like watching paint dry or kettles boil) and I was thinking 'if the RA dial marked 0-24 hrs goes round in less than 24 hrs, something must be wrong'.

So 1 hour on the RA dial goes past in just under 1 sidereal hour, so a sidereal clock still reads 24 hours a day, but runs fast by 4 minutes a day.

<I need a cold shower>

[Gazabone]

I think think you've summed it up better than my attempt!

[dph1nm]

so a sidereal clock still reads 24 hours a day, but runs fast by 4 minutes a day.

... which is why we see different constellations at different times of year.

NigelM

[ronin]

I suspect that the truth is the other way, 24 sidereal hours is one day or one rotation of the earth.

One solar day is a bit longer as the earth has to complete 1 complete rotation then has to rotate a little more to allow for it's orbital rotation component.

The "problem" arises as we generally would say that 1 day is the period from the sun crossing the Greenwich Meridian one day until it crosses it the next. That is incorrect, it takes a little more for the sun to be at the meridian again as the earth has moved a little more around the sun and this orbital motion/passage has to be taken into effect.

The catch is we are just used to whatever the clocks say, and even more so after we adopted one time over the UK/England for the train timetables.

What do you want to define as a "day"?

One rotation of the earth, or, sun over the meridian ?

The situation being that 1 complete and exact rotation of the earth is not the same period as the sun crossing the meridian on consequative "days".

Your watch, clock, PC, Radio and TV use the sun over the meridian option, the RA drive uses 1 complete rotation of the earth.

Trouble arises as they are very close to each other in value and they use the same terms = days, hours etc.

Technically I suppose it is incorrect to say that the earth rotates in 1 day where we measure time in solar days.