<p>@Teemu had the right idea: It's time zones. The specified location is in Kolkata, formerly Calcutta. Years before the adoption of India Standard Time (UTC+5:30), Kolkata had its own time zone, Calcutta Time (UTC+5:53:20). The 23+ minute difference between the two nicely accounts for the 23 minute difference between 6:42 and 6:19 in the reported sunrise time in 1901 versus now.</p>
<p>Now what about the calculations for 1902 and 1905? There the difference is 9 minutes rather than 23 minutes, and the difference is the other way around. That means either than Kolkata had briefly switched from the Calcutta Time Zone to the Madras Time Zone (UTC+05:21:14) in that time frame, or that the NOAA Solar Calculator thought they did. This nicely accounts for the 9 minute difference between 6:19 and 6:10.</p>
<p>This also nicely jibes with the fact that it was railroads worldwide that drove the adoption of time zones, and this happened during the mid to late 1800s and into the early 1900s. Time was at first transmitted manually via accurate chronometers, then via telegraph, then via radio, and now by the internet. Madras Time was the time standard briefly pushed India-wide by India's railroad industry. The Indian railroad industry later joined the push for adopting India Standard Time.</p>

CC BY-SA 4.0
2024-12-01T17:19:28.450Z
2024-12-02T14:45:17.770Z
2024-12-02T14:45:17.770Z
David Hammen
<p>According to NOAA Solar Calculator, the sunrise times on 12th January at latitude 22.532853 and longitude 88.3740234 are as follows over the years. Can you please explain the sudden jumps? Thanks.
1863 - 6:42
1901 - 6:42
1902 - 6:10
1905 - 6:10
1906 - 6:19
2024 - 6:19</p>

CC BY-SA 4.0
2024-11-30T08:33:48.583Z
Not a full answer. What is going on is that the calculator knows something about time zones. India used to have competing time zones before the standardization to Madras time which we now have, but a quick search claims the standardization happened in 1905, not 1902. But it was in reality a gradual progress, and different cities changed at different times, with 1905 marking the eventual end state.
Teemu Kalvas
CC BY-SA 4.0
2024-11-30T15:27:13.133Z
Look for a note designating what time zone the times refer to.
Greg Miller
CC BY-SA 4.0
2024-11-30T18:17:25.953Z
India had (still had?) 30 minute time zones. Then various leap seconds adding up.
Jon Custer
CC BY-SA 4.0
2024-11-30T19:39:07.720Z
Don't forget about continental drift  :-). The 32 min difference sounds like a time zone change, but the 9 min difference is odd.
JohnHoltz
CC BY-SA 4.0
2024-11-30T22:05:04.953Z
Max predictions of continental drift are about 10cm/yr, over 100 years that's about 30 feet.  But 9 minutes of Earth rotation (at the equator) is about 135 miles.
Greg Miller
CC BY-SA 4.0
2024-12-01T03:11:09.230Z
@TeemuKalvas You should make that an answer. Kolkata (previously know as Calcutta) used to have Calcutta Time, which was UTC+5:53:20 while India Standard Time is UTC+05:30. That accounts for the 23 minute difference between 6:42 and 6:19. What happened in 1902, who knows? It is just time zone changes.
David Hammen
<the-sun><history><positional-astronomy><sunrise>
Why does the sunrise time at a particular location at a particular date change abruptly over the years?
A Biswas
# Why does the sunrise time at a particular location at a particular date change abruptly over the years?

(Asked by: username_0 on 2024-11-30)

According to NOAA Solar Calculator, the sunrise times on 12th January at latitude 22.532853 and longitude 88.3740234 are as follows over the years. Can you please explain the sudden jumps? Thanks.
1863 - 6:42
1901 - 6:42
1902 - 6:10
1905 - 6:10
1906 - 6:19
2024 - 6:19

**Tags:** the-sun, history, positional-astronomy, sunrise
> **Comments:**
> **username_1**: Not a full answer. What is going on is that the calculator knows something about time zones. India used to have competing time zones before the standardization to Madras time which we now have, but a quick search claims the standardization happened in 1905, not 1902. But it was in reality a gradual progress, and different cities changed at different times, with 1905 marking the eventual end state.  
> **username_2**: Look for a note designating what time zone the times refer to.  
> **username_3**: India had (still had?) 30 minute time zones. Then various leap seconds adding up.  
> **username_4**: Don't forget about continental drift  :-). The 32 min difference sounds like a time zone change, but the 9 min difference is odd.  
> **username_2**: Max predictions of continental drift are about 10cm/yr, over 100 years that's about 30 feet.  But 9 minutes of Earth rotation (at the equator) is about 135 miles.  
> **username_5**: @TeemuKalvas You should make that an answer. Kolkata (previously know as Calcutta) used to have Calcutta Time, which was UTC+5:53:20 while India Standard Time is UTC+05:30. That accounts for the 23 minute difference between 6:42 and 6:19. What happened in 1902, who knows? It is just time zone changes.  

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## 1 answers

### Answer by username_5 on 2024-12-01. Score: 7 | Accepted answer

@Teemu had the right idea: It's time zones. The specified location is in Kolkata, formerly Calcutta. Years before the adoption of India Standard Time (UTC+5:30), Kolkata had its own time zone, Calcutta Time (UTC+5:53:20). The 23+ minute difference between the two nicely accounts for the 23 minute difference between 6:42 and 6:19 in the reported sunrise time in 1901 versus now.

Now what about the calculations for 1902 and 1905? There the difference is 9 minutes rather than 23 minutes, and the difference is the other way around. That means either than Kolkata had briefly switched from the Calcutta Time Zone to the Madras Time Zone (UTC+05:21:14) in that time frame, or that the NOAA Solar Calculator thought they did. This nicely accounts for the 9 minute difference between 6:19 and 6:10.

This also nicely jibes with the fact that it was railroads worldwide that drove the adoption of time zones, and this happened during the mid to late 1800s and into the early 1900s. Time was at first transmitted manually via accurate chronometers, then via telegraph, then via radio, and now by the internet. Madras Time was the time standard briefly pushed India-wide by India's railroad industry. The Indian railroad industry later joined the push for adopting India Standard Time.


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