# Sun Transit Time

1. Oct 2, 2004

### Jonstar

Well, i know that sun transit time is when the center of the sun crosses an imaginary line (the meridian) in the sky of the observer (or the given city). But, what i'm not understanding is why the sun transit time is different when we compare Toronto and Boston. They are both relatively close and in the same hemisphere, but why is it different!?

2. Oct 3, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums Jonstar!

See if you can find the longitude of each city ... are they the same? If the Earth is rotating about an axis through the poles, and longitude is (well, what is it?), then when would something like the Sun appear to cross the meridian, for two observers at the same longitude (but different latitudes)? at different longitudes?

3. Oct 4, 2004

### tony873004

Until the invention of the telegraph, both Boston and Toronto probably did have local noon at the Sun-meridian transit time. People everywhere just set their clocks so that the Sun crossing the meridian happened at 12 noon. They were not concerned that someone who lived 100 miles East of them had a clock that ran 15 minutes faster or slower than their clock.

But soon after people could instantly communicate over large distances, time zones were invented because it didn't make sense for someone in San Francisco to say to somebody in Sacramento that it was 10:32 when the person in Sacramento insisted that it was 10:43.

With time zones, people up to a few hundred miles on either side of you would share the same time zone. Only people in the middle of the time zone get to have the Sun cross the meridian at noon. Everyone else gives or takes a little to conform to the time zone. Imagine standing right at the sign that says "Entering Moutain Time Zone". Imagine looking up at the Sun at noon, then stepping across the line. The Sun wouldn't all of a sudden jump forwards or backwards one hour in the sky. You just moved from a place that experienced its local noon about 1/2 hour before the Sun-crossing of the meridian, to a place that experiences it about 1/2 hour after the crossing. Toronto and Boston have way different longitudes even though they're in the same time zone. So with their clocks sychronized to the same time, the Sun will cross the meridian at different times for them.

4. Oct 5, 2004

### Garth

The arrival of the railways also caused people to want specific time zones, before everytime they stepped off their trains they found the station clock was wrong!

5. Oct 5, 2004

### turbo

Also remember the safety aspect. Early on, there was only one track to most locations and few sidings. If train A is supposed to get on a siding and stay there until oncoming train B passes, and that doesn't happen in a timely fashion....well, the results are not pretty. Steam boilers can be pretty dangerous pieces of equipment, and when you put them on wheels and crash them head-on people will get hurt. Those really expensive "conductor's watches" were vital pieces of safety equipment.