# How many minutes does a sundial change every day?

I'm planning to create a sundial in Ghana (10° 28.852', 0° 26.032') in July 2014. I will stay there for about 3 weeks. Do I need to adjust the numbers anywhere in these 3 weeks? I know the shadow's path changes through the year, but how much does it change? Is the change significant for a sundial?

(btw, is this the correct site to ask or should I use the astronomy site?)

## 2 Answers

You need to look up "the equation of time" - this is a correction to the time reported by the sun on any given day - the apparent solar time - (and measured by your sundial) and the standard civil time - the mean solar time - it varies throughout the year and the maximum your sundial will be out is about 15 minutes - but the difference and the rate of change varies. http://en.wikipedia.org/wiki/Equation_of_time

If your sundial is merely for educative/demonstration purposes, this might not matter so much of course - as the sundial will surely demonstrate that the sun's shadow can be used to tell the time.

• My sundial will be use to tell the time approximately. A 15 minutes difference doesn't really matter.
– user737
Jan 20 '14 at 10:14

According to the wiki entry, the error runs up to roughly 15 minutes. One can use the "equation of time" , for which there appear to be published tables, to generate a correction curve. (The main contributors to the error are eccentricity and obliquity of Earth's orbit.

• By 15 minutes, they mean 15 in a year, I guess? That will be equal to just under a minute in 3 weeks.
– user737
Jan 9 '14 at 18:58
• No, they mean the maximum difference between local time and the time on your sundial will be 15 minutes. Since the sun changes its apparent position over the year, the time difference between the sundial and local time changes too. It's not a cumulative effect, like a clock winding down. Jan 30 '14 at 7:04