# The Naked Scientists Forum

### Author Topic: Time  (Read 3207 times)

#### syhprum

• Neilep Level Member
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##### Time
« on: 12/11/2006 12:55:55 »
I want to set my sundial to UTC, what time and date will the centre of the sun be due south in the UK.
I am located at 51.778 north 0.44 west

#### jysk

• Jr. Member
• Posts: 41
##### Re: Time
« Reply #1 on: 12/11/2006 18:57:24 »
Hi Syhprum,

Your problem is interesting. It seemed that the sun's position is "Due South" at the time midway between sun rise and sun set only at equinox. Crazy calculations would be needed to predict that postion at other times.

But now I'm thinking that "Due South" is just the point at which the sun is highest in the sky on any day. The highest point in the sky is entirely due to the Earth's spin on it's North-South axis. I think it's called the celestial meridian, and it occurs predictably everywhere on Earth.

But just to be safe, don't order your concrete until March 21st.

Mike

#### syhprum

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##### Re: Time
« Reply #2 on: 12/11/2006 19:08:19 »
It is easy to make the adjustment for the longitude (about 1.46.6) but I do not know how to adjust for the latitude or whether the the earths rotational period should be taken as 86400 seconds or 86163.45 secs

#### syhprum

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##### Re: Time
« Reply #3 on: 12/11/2006 19:09:17 »
Should I not choose December 21 ?

#### jysk

• Jr. Member
• Posts: 41
##### Re: Time
« Reply #4 on: 12/11/2006 19:29:38 »
In addition, to calculate the solar centre, I'd just find out what part of the disk is forcasted to rise and set. What I mean is, I'd ask; "Is sunrise defined as the time the top of the disk peeks over the horizon?" Find out how sunset is defined and then predict the exact moment in between.

Set your sundial at that moment.

It will be accurate exactly once every 4 years.

Mike

#### jysk

• Jr. Member
• Posts: 41
##### Re: Time
« Reply #5 on: 12/11/2006 19:50:48 »
I'm sorry I seemed rude, but I didn't see your two posts before I added my second.

To pick up from your first response, I don't believe that your local coordinates matter at all. The sun is due south at the moment it is at it's highest point in the sky.

I can complicate it though.

There might be some declination due to the sun moving slightly up or down through that highest point in the sky (more dramatically in spring and fall), and I guess the sun might not be a perfect sphere, but wow, I can't imagine physically measuring that. That declination would certainly be an interesting mathematic exercise.

Mike
« Last Edit: 12/11/2006 20:06:04 by jysk »

#### syhprum

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##### Re: Time
« Reply #6 on: 12/11/2006 20:24:20 »
yes of course when I have confirmed that the Sun is at it highest point it will be in the South, knowing what the UTC time is would enable me to compute my longitude, I take it that my latitude would be irrelevant.
It is very easy to obtain UTC these days, I use Satscape

www.satscape.co.uk

#### Mjhavok

• Sr. Member
• Posts: 468
##### Re: Time
« Reply #7 on: 12/11/2006 23:58:46 »