THE BASIC CALCULATION
Background
All you need to know is your Longitude to determine when solar noon (the sun being the highest in the sky for that day*) is. Since there are 24 hours in a day its obvious we need 24 time zone to make our lives run more smoothly and “correct” for the sun’s motion around the earth to match our wrist watch time. Time Zones were created so that the clock time where we live would correspond more closely with the sun’s movements (the sun rising when our wrist watch says its 6 or 7 AM and the sun setting when our wrist watch says its 8 or 9 PM in the evening). With a globle having 360 degrees and there being 24 hours in a day it was pretty simple to see that there would have to be 24 time zones and each time zonewould be 15 degreses wide (360 degrees /24 hours = 15 degrees per time zone).
The Beginning of Your Time Zone
When the times zones were created the designers picked a place in England as a starting point (you have to start somewhere. That place was Greenwich, England , which they designated as 0 degrees and the start of the first Time Zone. When the sun was directly overhead in Greenwich England they called that time 12:00 noon. One hour later the sun would be directly overhead in another spot on the earth…15 degrees of longitude to the east of Greenwich,England so at that spot it was the beginning of the next Time Zone and solar noon there would be when the sun was at its highest point and our wrist watch pointed to 12:00 noon.
To summarize…at the beginning of each time zone the sun is at solar noon (the highest point its going to get during that day) when your wrist watch is pointing to 12 noon.. The beginning of each new time zone is 15 degree from the last one. Each Time Zone starting point is at the set latitudes of… 0 degrees, 15 degrees, 30, 45, 60, 75, 90, 105, 120, 135, 150 degrees ect.
Calculating How Many Minutes to Add
For those that are lucky enough to live RIGHT ON on of those longitudes lines then solar noon will be when your wrist watch is pointing to the 12:00. However most of us do not live right on one of those longitude line (0, 15, 30, 45 ect) so we need to correct for the distanvce we are away from that longitude line that starts our Time Zone. With 60 minutes in each hour and 15 degress in a time zone then for every 1 degree we live tio the west of one of the starting point of out Time Zone, solar noon will be 4 minutes later than the exact 12:00 our wrist watch indicates. (60 minutes / 15 degrees = 4 minutes per degree.).
So to calculate solar noon where you live just figure out how many degrees you are west from the last time zone starting point and add 4 minutes for each degree you are away from it!
Example – Time Zone Beginning
You know that the Time Zones are at set degrees (0, 15, 30, 45 ect.) so if you live at longitide 38 degree the start of your Time Zone was at 30 degrees.
If you live at longitude 108 degrees, the the start of your Time Zone was at 105 degrees
If you live at longitude 118 degrees, the start of your Time Zone is still 105 degrees (your lovcation is not past the next Time Zone start at 120 degrees)
Example – Calculating “Solar Noon”
If I lived in Cleveland, Ohio (longitude 81.5 degrees)
The beginning of my Time Zone (Eastern Standard Time) was at 75 degrees, so Cleveland, Ohio at 81.5 degrees of latitude is 6.5 degrees to the west of my Time Zone’s starting point
6.5 degrees x 4 minutes per degree = 26 minutes
So, solar noon in Cleveland, Ohio is the sum of 12:00 + 26 minutes = 12:26 PM
- Fun Fact: The sun is never DIRECTLY overhead in the United States, its always south of “directly overhead” when it is at its highest point in the sky, even in mid-summer.
This calculation will give you a prety close estimate when “solar noon” will be however to be more precise we need to add in a few correction…
CORRECTION #1 – Daylight Savings Time
Most all of us are on Daylight Savings Time for about5 months per year and are familar with the rhyming reminder of “spring frowar and fall back” to indicate which way we move the hands of the clock when its time to go from standard time to daylight savings time and then back again. If you are making your “high noon” calculations when Daylight Savings Time is in effect you must ADD 1 hour to the your calculated solar noon” time.
In the above example if it were July 1, Daylight Savings Time is in effect, so solar noon would occure at 1:26 PM (12:26 + 1 hour for DST)= 1:26 PM
CORRECTION #2 – “Equation of Time”
Two other things also affect EXACTLY when solar noon will occur. They are::
- the earth’s 23.4 degree tilt on its axis
- the earths eccentric (not perfectly circular) orbit around the sun
The equation of time is defined as the offset between wrist watch time and solar time. Solar time is judged by the position of the Sun in the sky, where midday is the moment when the Sun appears highest in the sky on every day of the year. Wrist watch time is just that, the time your watch displays.
Most every day of the year your basic calculations will be off only a few minutes if you do not take the earth’s tilt and orbital eccentricity into account but it can be as much as 14 minutes in early February. There are however 4 days that the calculated time and wrist watch time match…they are April 15, June 12, September 1 and December 25. On all of the othere days, to be more precise, we need to either add or subtract a few minutes to our calculate “solar noon” time.
Use the chart below to either add or subrtract a few monues to get your calculations even more exact and correct for the “Equation of Time” deviations.
In our above example if it were July 1, the chart below indicates the Equation of Time correction factor is -3 min 56 sec. AND with Daylight Savings Time in effect our “solar noon” would occure at 1:26 PM – 3 min 56 sec. or 1:22 and 4 sec. PM
“Equation of Time” Corretion Table
Red – subrtract from longitude calculated solar noon
Green – add to longitude calculated solar noon
Date Min. Sec.
Jan 01: -3m 41s
Jan 02: -4m 9s
Jan 03: -4m 37s
Jan 04: -5m 3s
Jan 05: -5m 30s
Jan 06: -5m 57s
Jan 07: -6m 23s
Jan 08: -6m 48s
Jan 09: -7m 13s
Jan 10: -7m 37s
Jan 11: -8m 1s
Jan 12: -8m 24s
Jan 13: -8m 47s
Jan 14: -9m 8s
Jan 15: -9m 30s
Jan 16: -9m 50s
Jan 17; -10m 10s
Jan 18: -10m 29s
Jan 19: -10m 48s
Jan 20: -11m 5s
Jan 21: -11m 23s
Jan 22: -11m 39s
Jan 23: -11m 54s
Jan 24: -12m 9s
Jan 25: -12m 23s
Jan 26: -12m 36s
Jan 27: -12m 48s
Jan 28: -12m 59s
Jan 29: -13m 10s
Jan 30: -13m 20s
Jan 31: -13m 29s
Feb 01: -13m 37s
Feb 02: -13m 44s
Feb 03
-13m 51s
Feb 04
-13m 57s
Feb 05
-14m 2s
Feb 06
-14m 6s
Feb 07
-14m 9s
Feb 08
-14m 11s
Feb 09
-14m 13s
Feb 10
-14m 14s
Feb 11
-14m 14s
Feb 12
-14m 13s
Feb 13
-14m 12s
Feb 14
-14m 10s
Feb 15
-14m 7s
Feb 16
-14m 3s
Feb 17
-13m 59s
Feb 18
-13m 54s
Feb 19
-13m 48s
Feb 20
-13m 42s
Feb 21
-13m 35s
Feb 22
-13m 27s
Feb 23
-13m 19s
Feb 24
-13m 10s
Feb 25
-13m 0s
Feb 26
-12m 50s
Feb 27
-12m 39s
Feb 28
-12m 28s
Feb 29
-12m 16s
Mar 01
-12m 16s
Mar 02
-12m 4s
Mar 03
-11m 52s
Mar 04
-11m 38s
Mar 05
-11m 25s
Mar 06
-11m 11s
Mar 07
-10m 56s
Mar 08
-10m 42s
Mar 09
-10m 27s
Mar 10
-10m 11s
Mar 11
-9m 55s
Mar 12
-9m 39s
Mar 13
-9m 23s
Mar 14
-9m 6s
Mar 15
-8m 49s
Mar 16
-8m 32s
Mar 17
-8m 15s
Mar 18
-7m 58s
Mar 19
-7m 40s
Mar 20
-7m 22s
Mar 21
-7m 4s
Mar 22
-6m 46s
Mar 23
-6m 28s
Mar 24
-6m 10s
Mar 25
-5m 52s
Mar 26
-5m 34s
Mar 27
-5m 16s
Mar 28
-4m 58s
Mar 29
-4m 40s
Mar 30
-4m 22s
Mar 31
-4m 4s
Apr 01
-3m 46s
Apr 02
-3m 28s
Apr 03
-3m 11s
Apr 04
-2m 54s
Apr 05
-2m 36s
Apr 06
-2m 19s
Apr 07
-2m 3s
Apr 08
-1m 46s
Apr 09
-1m 30s
Apr 10
-1m 14s
Apr 11
-0m 58s
Apr 12
-0m 43s
Apr 13
-0m 27s
Apr 14: -0m 13s
Apr 15: 0m 2s
Apr 16 0m 16s
Apr 17 0m 30s
Apr 18 0m 43s
Apr 19 0m 56s
Apr 20 1m 9s
Apr 21 1m 21s
Apr 22 1m 32s
Apr 23 1m 44s
Apr 24 1m 54s
Apr 25 2m 5s
Apr 26 2m 14s
Apr 27 2m 24s
Apr 28 2m 32s
Apr 29 2m 41s
Apr 30 2m 48s
May 01 2m 56s
May 02 3m 2s
May 03 3m 8s
May 04 3m 14s
May 05
3m 19s
May 06 3m 24s
May 07 3m 27s
May 08 3m 31s
May 09 3m 34s
May 10 3m 36s
May 11 3m 37s
May 12 3m 38s
May 13 3m 39s
May 14 3m 39s
May 15 3m 38s
May 16 3m 37s
May 17 3m 35s
May 18 3m 33s
May 19 3m 30s
May 20 3m 26s
May 21 3m 22s
May 22 3m 18s
May 23 3m 13s
May 24 3m 7s
May 25 3m 1s
May 26 2m 55s
May 27 2m 48s
May 28 2m 40s
May 29 2m 33s
May 30 2m 24s
May 31 2m 16s
Jun 01 2m 6s
Jun 02 1m 57s
Jun 03 1m 47s
Jun 04 1m 37s
Jun 05 1m 26s
Jun 06 1m 15s
Jun 07 1m 4s
Jun 08 0m 53s
Jun 09 0m 41s
Jun 10 0m 29s
Jun 11 0m 17s
Jun 12 0m 4s
Jun 13
-0m 8s
Jun 14
-0m 21s
Jun 15
-0m 34s
Jun 16
-0m 47s
Jun 17
-0m 60s
Jun 18
-1m 13s
Jun 19
-1m 26s
Jun 20
-1m 39s
Jun 21
-1m 52s
Jun 22
-2m 5s
Jun 23
-2m 18s
Jun 24
-2m 31s
Jun 25
–
2m 43s
Jun 26
-2m 56s
Jun 27
-3m 8s
Jun 28
-3m 21s
Jun 29
-3m 33s
Jun 30
-3m 45s
Jul 01
-3m 56s
Jul 02
-4m 8s
Jul 03
-4m 19s
Jul 04
-4m 29s
Jul 05
-4m 40s
Jul 06
-4m 50s
Jul 07
-4m 60s
Jul 08
-5m 9s
Jul 09
-5m 18s
Jul 10
-5m 26s
Jul 11
-5m 34s
Jul 12
-5m 42s
Jul 13
-5m 49s
Jul 14
-5m 56s
Jul 15
-6m 2s
Jul 16
-6m 7s
Jul 17
-6m 12s
Jul 18
-6m 17s
Jul 19
-6m 21s
Jul 20
-6m 25s
Jul 21
-6m 27s
Jul 22
-6m 30s
Jul 23
-6m 31s
Jul 24
-6m 33s
Jul 25
-6m 33s
Jul 26
-6m 33s
Jul 27
-6m 33s
Jul 28
-6m 31s
Jul 29
-6m 30s
Jul 30
-6m 27s
Jul 31
-6m 24s
Aug 01
-6m 20s
Aug 02
-6m 16s
Aug 03
-6m 11s
Aug 04
-6m 6s
Aug 05
-5m 60s
Aug 06
-5m 53s
Aug 07
-5m 46s
Aug 08
-5m 38s
Aug 09
-5m 29s
Aug 10
-5m 20s
Aug 11
-5m 11s
Aug 12
-5m 1s
Aug 13
-4m 50s
Aug 14
-4m 39s
Aug 15
-4m 27s
Aug 16
-4m 15s
Aug 17
-4m 2s
Aug 18
-3m 49s
Aug 19
-3m 35s
Aug 20
-3m 20s
Aug 21
-3m 6s
Aug 22
-2m 51s
Aug 23
-2m 35s
Aug 24
-2m 19s
Aug 25
-2m 2s
Aug 26
-1m 45s
Aug 27
-1m 28s
Aug 28
-1m 11s
Aug 29
-0m 53s
Aug 30
-0m 34s
Aug 31
-0m 16s
Sep 01 0m 3s
Sep 02 0m 23s
Sep 03 0m 42s
Sep 04 1m 2s
Sep 05 1m 22s
Sep 06 1m 42s
Sep 07 2m 3s
Sep 08 2m 23s
Sep 09 2m 44s
Sep 10 3m 5s
Sep 11 3m 26s
Sep 12 3m 47s
Sep 13 4m 8s
Sep 14 4m 30s
Sep 15 4m 51s
Sep 16 5m 13s
Sep 17 5m 34s
Sep 18 5m 55s
Sep 19 6m 17s
Sep 20 6m 38s
Sep 21 6m 59s
Sep 22 7m 21s
Sep 23 7m 42s
Sep 24 8m 3s
Sep 25 8m 23s
Sep 26 8m 44s
Sep 27 9m 5s
Sep 28 9m 25s
Sep 29 9m 45s
Sep 30 10m 5s
Oct 01 10m 24s
Oct 02 10m 43s
Oct 03 11m 2s
Oct 04 11m 21s
Oct 05 11m 39s
Oct 06 11m 57s
Oct 07 12m 15s
Oct 08 12m 32s
Oct 09 12m 48s
Oct 10 13m 4s
Oct 11 13m 20s
Oct 12 13m 35s
Oct 13 13m 50s
Oct 14 14m 4s
Oct 15 14m 18s
Oct 16 14m 31s
Oct 17 14m 43s
Oct 18 14m 55s
Oct 19 15m 6s
Oct 20 15m 17s
Oct 21 15m 27s
Oct 22 15m 36s
Oct 23 15m 45s
Oct 24 15m 53s
Oct 25 15m 60s
Oct 26 16m 6s
Oct 27 16m 12s
Oct 28 16m 17s
Oct 29 16m 21s
Oct 30 16m 24s
Oct 31 16m 27s
Nov 01 16m 28s
Nov 02 16m 29s
Nov 03 16m 29s
Nov 04 16m 29s
Nov 05 16m 27s
Nov 06 16m 25s
Nov 07 16m 21s
Nov 08 16m 17s
Nov 09 16m 12s
Nov 10 16m 6s
Nov 11 15m 60s
Nov 12 15m 52s
Nov 13 15m 44s
Nov 14 15m 34s
Nov 15 15m 24s
Nov 16 15m 13s
Nov 17 15m 1s
Nov 18 14m 49s
Nov 19 14m 35s
Nov 20 14m 21s
Nov 21 14m 6s
Nov 22 13m 50s
Nov 23 13m 33s
Nov 24 13m 16s
Nov 25 12m 57s
Nov 26 12m 38s
Nov 27 12m 19s
Nov 28 11m 58s
Nov 29 11m 37s
Nov 30 11m 15s
Dec 01 10m 53s
Dec 02 10m 30s
Dec 03 10m 6s
Dec 04 9m 42s
Dec 05 9m 17s
Dec 06 8m 52s
Dec 07 8m 26s
Dec 08 7m 60s
Dec 09 7m 33s
Dec 10 7m 6s
Dec 11 6m 38s
Dec 12 6m 10s
Dec 13 5m 42s
Dec 14 5m 13s
Dec 15 4m 45s
Dec 16 4m 16s
Dec 17 3m 46s
Dec 18 3m 17s
Dec 19 2m 47s
Dec 20 2m 18s
Dec 21 1m 48s
Dec 22 1m 18s
Dec 23 0m 49s
Dec 24 0m 19s
Dec 25
-0m 11s
Dec 26
-0m 40s
Dec 27
-1m 10s
Dec 28
-1m 39s
Dec 29
-2m 8s
Dec 30
-2m 37s
Dec 31
-3m 6s