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Astronomy GCSE: Sundial Coursework help

With the coursework I am meant to use a sundial and test its accuracy. I have found Polaris and have aligned my gnomon with it. If my sundial is accurate, should my equation of time for today be 5.4? (http://www.wsanford.com/~wsanford/exo/sundials/equation_of_time.html)

If it isn't it can be any other value and this would tell me how accurate my sundial is? Am I correct in my understanding of this coursework?

My sundial is a properly manufactured one so it should be accurate. At 12:55GMT, my sundial showed a time of 13:02pm does this mean that my sundial is correctly aligned, etc(This means my eq.of.time is +7, is this due to my alignment of the sundial?)

I'm really confused. :confused:

(edited 11 years ago)

Reply 1

ThatPerson

Also, I am meant to take 6 readings spread out over at least 3 weeks. I'm starting today, so how many days gap should I leave between each reading? I was thinking 5 days but is this too much? Would 3 days be OK? (I'd take more readings, but I'd also have more data).

Reply 2

martianfrog

Take your readings as spaced apart as possible (months rather than days) when the equation of time correction is very different.

Don't forget you need a longitude correction too (that might be one cause of your difference being 7 instead of 5.4 - are you 0.4 degrees west of greenwich?).

Consider how you are taking your measurements - how accurate are they? Are you reading leading or trailing edge for example? If you took several readings today (5 or 6 say) would they all be exactly 7 mins out? The range of such a set of readings would show you what the reading error on your sundial might be (I think it's typically 2-3 mins, so could easily be another cause for your discrepancy). You'd then need some more widely spaced readings to get the sundial accuracy.

Your sundial probably isn't brilliantly aligned - very hard to judge when it's pointing exactly to polaris, and polaris isn't even exactly on the celestial pole anyway (but it is close), so there's another possible source of error.

Reply 3

ThatPerson

Original post by martianfrog

My coursework must be completed over the summer holidays which is a period of weeks. I have a 4 weeks to complete this in(I did take a reading 5 days ago but I later realised I had done it wrong). I am east of Greenwich so I guess my sundial is aligned slightly wrong.

I don't think my teacher mentioned anything about Longitude correction but I guess if needed I can do that retrospectively.

Reply 4

martianfrog

Well spread your readings out as much as you can, but I would also take a set on one day as well to try and work out the reading error. Subtract 4 minutes for each degree east of Greenwich that you are (basically local noon is earlier the further East you are).

The controlled assessment doesn't have to be submitted until next May, so it's your teacher who is setting arbitrary timescales, but as she's the one who will presumably be supervising you writing it up, I suppose that's her call.

Good luck.

Reply 5

Drummy

work out your local mean time at 12;00 gmt (1:00 bst) on a certain day

you need to know the eot on that day and your longitude to do this

now compare this with what the sundial says at 12:00 on that day to determine its accuracy

do this on three days at least a few weeks apart

Reply 6

ThatPerson

Original post by martianfrog

Just to check, the sundial time and my normal clock time are supposed to have differences, so the fact that my sundial is +4.5 ahead actually means that it is accurate and correctly aligned to Polaris?

Reply 7

Drummy

Do you know what the equation of time is?

Have a look on the relevant pages on

www.astronomygcse.co.uk

and have a look at

http://www.youtube.com/watch?v=rZtZ7xQjxhE

It will explain why there is a difference between your apparent solar time and GMT

Reply 8

ThatPerson

Original post by Drummy

I have a slight question. Here is the table:

http://www.wsanford.com/~wsanford/exo/sundials/equation_of_time.html

So assuming in standard time it is 12:00, for the 1st of January, does this mean the time on the sundial is 12:03.4, or does this mean that the sundials' time would be 12.56.6 and I would have to add the 3.4 to get the standard time?

Please help :confused: