Earth's Rotation & Longitude

Question

I live in Townsville Australia which has a longitude of 146 degrees East. I'm trying to get my head around how long the Sun takes to move from Greenwich to my latitude. I have a world globe in front of me to help me understand. If the sun moves West to East and moves 15 degrees across the sky in one hour, it would need to move 210 degrees from Greenwich to Townsville which equates to a little over 14 hours. This can't be correct as Townsville time is 10 hours ahead of Greenwich time. I'm obviously missing something in my thinking. Can anyone help me out?

Answer 1

Both is correct. It just depends which way you look.

Greenwich is by definition at longitude 0°. As the Sun moves 15° per hour from East to West. You can simply do the equation if you know your longitude. The time the sun reaches your local meridian (noon) before it does the same at Greenwich is: $$ \Delta t = \frac{\lambda_{place} - \lambda_{greenwich}}{15°/h}$$

So basically divide your longitude by 15 and you get the difference in hours. You should see the sun on your meridian (due north) 9.73 hours (or 9h:44) before it reaches the meridian of Greenwich. You see the sun on the meridian before that happens at Greenwich for every longitude Eastern of Greenwich and hours after Greenwich for longitudes Western of Greenwich.

Or if you want to do it the other way around, add 24 hours: 24h - 9:44h = 14:16h after Greenwhich (but that's already the next day for you).

Answer 2

Your location is west of the International Date Line. The time of sunrise gets later and later in terms of UTC time as one's location moves to the west from the Greenwich meridian because the Sun appears to move from east to west. But then one crosses the International Date Line at roughly 180° longitude and in an instant it's suddenly the next day.

There's nothing magical happening here that suddenly makes it the next day on crossing the International Date Line going west, or suddenly makes it the previous day on crossing the International Date Line going east. It's just a consequence of how we keep time around the world. You can look at your location as being 14 hours and 16 minutes behind Greenwich, or thanks to the International Date Line, as being 9 hours and 44 minutes ahead of Greenwich. They're the same thing.

Answer 3

You are 10 hours ahead of Greenwich... and so you have the sun overhead 10 hours before Greenwich roughly. Which is about the 146° between your locations (in the Eastern Hemisphere), divided by 15°. And that's why your timezones are 10 hours different.

Then when the sun is overhead in Greenwich, it needs to travel the rest of the globe round to get back to you... which indeed is about the 210° you calculated: it would need to traverse the full 180° of the Western Hemisphere continuing west from Greenwich, plus the additional 34° you are from the Date Line, before it gets back to you. So there are indeed 14 time zones going from Greenwich west to Townsville.

So you mixed two different things, calculating the longer angle, but using the shorter time difference, and so getting a confusing result. Both are true, but they are found going different directions.