Solar Longitude Calculator Guide
Find Solar Time & Time Correction by Location Calculate your standard meridian, solar time correction, local solar time and equation of time for accurate sun position and solar panel performance by longitude.
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Solar Longitude Calculator Guide
Results use standard solar time equations (Spencer 1971, Duffie & Beckman).
Daylight Saving Time is not automatically detected — adjust UTC offset manually during DST.
For precision solar engineering, use NREL Solar Position Algorithm (SPA).
How to Use Solar Longitude Calculator Guide
Get precise solar time corrections and LST conversions in just a few quick steps:
- Enter Your Longitude. Input your location longitude in decimal degrees. Use negative values for West longitudes and positive for East. Example: Chicago is -87.6, London is -0.1, Delhi is +77.2.
- Enter UTC Offset. Input your time zone offset from UTC. USA Eastern Standard Time is -5, Central is -6, Mountain is -7, Pacific is -8. During Daylight Saving Time add 1 hour to each offset.
- Select the Date. Choose the date for the calculation. The Equation of Time varies throughout the year, so the date affects the solar time correction by up to 16 minutes.
- Enter Local Clock Time. Input the local clock time you want to convert to Local Solar Time. Enter 12:00 to find the exact clock time of solar noon at your location.
- Click Calculate. Press Calculate Solar Time and Longitude to view your standard meridian, longitude correction, equation of time, total time correction, local solar time, solar noon and solar declination.
- Read Your Results. Use the Local Solar Time result for accurate sun position calculations. Use Solar Noon to know the exact clock time when the sun is highest and shadows are shortest at your location.
How to Calculate Solar Time Correction by Longitude Guide
What Is Solar Longitude and Why Does It Matter?
Your geographic longitude determines the difference between clock time and true solar time at your location. Clock time is set by time zones covering 15° of longitude each, meaning all locations within the same zone share one clock time regardless of their actual longitude. True solar time, called Local Solar Time, is based on the sun's actual position and is what matters for solar energy calculations, sun position modeling and panel orientation.
Step 1 — Find Your Standard Meridian
Each time zone is centered on a standard meridian, which is the longitude where clock time exactly matches solar time (ignoring the Equation of Time). Multiply your UTC offset by 15 to find your standard meridian.
Example (UTC -6 Central Time): Standard Meridian = -6 × 15 = -90°
Step 2 — Calculate Longitude Correction
If your longitude differs from your standard meridian, your solar time differs from clock time. Every 1° of longitude equals 4 minutes of time difference.
Example (Chicago longitude = -87.6°, Standard Meridian = -90°):
LC = 4 × (-90 − (-87.6)) = 4 × (-2.4) = -9.6 minutes
Chicago solar time is 9.6 minutes behind clock time due to longitude position.
Step 3 — Calculate Equation of Time
The Equation of Time (EoT) accounts for the elliptical orbit of Earth and axial tilt, which cause the sun to run up to 16 minutes fast or slow compared to a uniform clock. It varies throughout the year.
EoT = 9.87 × sin(2B) − 7.53 × cos(B) − 1.5 × sin(B) [minutes]
Range: −16.4 minutes (early November) to +14.3 minutes (mid-February)
Step 4 — Calculate Total Time Correction
TC = 4 × (Standard Meridian − Longitude) + EoT
Example (LC = -9.6 min, EoT = -3.0 min on Jan 1): TC = -9.6 + (-3.0) = -12.6 minutes
Step 5 — Convert Clock Time to Local Solar Time
Example (Clock = 12:00 decimal 12.0, TC = -12.6 min):
LST = 12.0 + (-12.6/60) = 11.79 hours = 11:47 LST
Step 6 — Find Solar Noon Clock Time
Solar noon is when LST = 12:00. Rearranging the formula gives the clock time of solar noon.
Example (TC = -12.6 min): Solar Noon = 12 + 12.6/60 = 12.21 hours = 12:13
Solar Longitude Reference Chart Guide
Use the tables below to find standard meridians, longitude corrections and equation of time values for major world cities and months of the year.
Standard Meridian and Longitude Correction by City
| City | Longitude | UTC Offset | Standard Meridian | Longitude Correction | Notes |
|---|---|---|---|---|---|
| New York, USA | -74.0° | -5 | -75° | +4.0 min | EST |
| Chicago, USA | -87.6° | -6 | -90° | -9.6 min | CST |
| Denver, USA | -104.9° | -7 | -105° | +0.4 min | MST — near meridian |
| Los Angeles, USA | -118.2° | -8 | -120° | -7.2 min | PST |
| London, UK | -0.1° | 0 | 0° | +0.4 min | GMT — near meridian |
| Paris, France | +2.3° | +1 | +15° | -51.2 min | CET — large correction |
| Dubai, UAE | +55.3° | +4 | +60° | -18.8 min | GST |
| Delhi, India | +77.2° | +5.5 | +82.5° | -21.2 min | IST |
| Beijing, China | +116.4° | +8 | +120° | -14.4 min | CST |
| Sydney, Australia | +151.2° | +10 | +150° | +4.8 min | AEST |
Equation of Time by Month
| Month | Approx. EoT (minutes) | Solar Noon Shift | Notes |
|---|---|---|---|
| January | +3.0 to +14.3 min | Solar noon early | Sun runs fast |
| February | +14.3 min (peak) | Solar noon earliest | Maximum fast (Feb 12) |
| March | +12.4 to 0 min | Decreasing | Crosses zero near equinox |
| April | −3.0 to −3.0 min | Solar noon late | Near zero crossing |
| May | −3.0 to +3.0 min | Near zero | Crosses zero (~May 15) |
| June | +2.0 min | Near normal | Small correction |
| July | +6.5 to −6.5 min | Decreasing | Crosses zero (~Jul 26) |
| August | −6.5 min | Solar noon late | Growing negative |
| September | −7.0 to 0 min | Decreasing | Crosses zero near equinox |
| October | −10.0 to −16.4 min | Solar noon latest | Growing negative |
| November | −16.4 min (peak) | Solar noon latest | Maximum slow (Nov 3) |
| December | −16.0 to +2.0 min | Recovering | Crosses zero (~Dec 25) |
Total Time Correction Examples (Longitude + EoT)
(For Chicago, longitude -87.6°, UTC -6, LC = -9.6 min)
| Month | EoT (min) | Longitude Correction | Total TC | Solar Noon (clock) |
|---|---|---|---|---|
| January 1 | -3.0 | -9.6 | -12.6 min | 12:13 |
| February 12 | +14.3 | -9.6 | +4.7 min | 11:55 |
| March 21 | +7.5 | -9.6 | -2.1 min | 12:02 |
| May 15 | +3.5 | -9.6 | -6.1 min | 12:06 |
| June 21 | -1.5 | -9.6 | -11.1 min | 12:11 |
| July 26 | +6.5 | -9.6 | -3.1 min | 12:03 |
| November 3 | -16.4 | -9.6 | -26.0 min | 12:26 |
| December 25 | +0.0 | -9.6 | -9.6 min | 12:10 |
Time Zones, Standard Meridians and UTC Offsets
| UTC Offset | Standard Meridian | Time Zone Examples | Region |
|---|---|---|---|
| -12 | -180° | Baker Island | Pacific |
| -8 | -120° | PST — Los Angeles | Americas West |
| -7 | -105° | MST — Denver | Americas Mountain |
| -6 | -90° | CST — Chicago | Americas Central |
| -5 | -75° | EST — New York | Americas East |
| 0 | 0° | GMT — London | Europe / Africa |
| +1 | +15° | CET — Paris, Berlin | Central Europe |
| +3 | +45° | AST — Riyadh | Middle East |
| +5.5 | +82.5° | IST — Delhi | South Asia |
| +8 | +120° | CST — Beijing | East Asia |
| +10 | +150° | AEST — Sydney | Australia East |
Solar Longitude Frequently Asked Questions Guide
Solar longitude refers to your geographic longitude and how it determines the relationship between clock time and true solar time at your location. Every 1° of longitude equals 4 minutes of time difference from your time zone's standard meridian. If your longitude differs from your standard meridian, the sun reaches its highest point either before or after 12:00 noon on your clock. Accurate solar time is essential for calculating sun position, panel orientation and expected energy output for solar installations.
Local Solar Time (LST) is the true time based on the sun's actual position in the sky, where 12:00 LST always corresponds to solar noon — the moment the sun is highest and due south (in the Northern Hemisphere). Clock time differs from LST because time zones cover 15° of longitude each and the Earth's orbit is not perfectly circular. Solar energy calculations, sun position formulas and panel shading analysis all require Local Solar Time for accurate results.
The Equation of Time (EoT) is the difference in minutes between true solar time and mean solar time (clock time) caused by two factors: Earth's elliptical orbit around the sun, which varies orbital speed throughout the year, and the axial tilt of 23.45°. These effects combine to make the sun run up to 16.4 minutes slow (in early November) or up to 14.3 minutes fast (in mid-February) compared to a uniform 24-hour clock. The Equation of Time must be added to longitude correction to get the total solar time correction.
A standard meridian is the central longitude of a time zone, equal to UTC offset multiplied by 15. For example, UTC -6 (US Central Time) uses the 90°W standard meridian. At the standard meridian, clock time exactly matches solar time (ignoring the Equation of Time). Locations east of the standard meridian have solar time ahead of clock time, while locations west have solar time behind clock time. The difference is 4 minutes per degree of longitude.
Solar noon is the clock time when the sun reaches its highest point and true south bearing at your location. To find it, calculate your total time correction (longitude correction plus Equation of Time) and subtract it from 12:00. For example, if your total correction is -15 minutes, solar noon occurs at 12:15 on your clock. Solar noon is critical for setting panel orientation, scheduling solar measurements and calculating day length for energy estimates.
Paris at longitude +2.3° sits in the UTC+1 time zone, whose standard meridian is +15°. The longitude difference of 12.7° creates a longitude correction of 4 × (15 − 2.3) = -50.8 minutes. This means true solar noon in Paris occurs around 12:50 to 13:10 depending on the Equation of Time correction for the date. This large correction is common across continental Europe because the CET time zone stretches far west of its standard meridian for political reasons.
Yes. During Daylight Saving Time (DST), clocks advance by 1 hour, which effectively increases your UTC offset by 1. For example, US Central Time changes from UTC-6 (CST) to UTC-5 (CDT) during summer. To use this calculator correctly during DST, add 1 to your UTC offset. The standard meridian and longitude correction change accordingly. Forgetting to adjust for DST will give solar noon times that are 1 hour too early.
Longitude affects solar panel performance indirectly through solar time accuracy. If a solar tracking system, monitoring software or energy simulation uses clock time instead of corrected Local Solar Time, predicted output can be off by up to 30 minutes of sun position, leading to incorrect tilt angles, shading predictions and energy yield calculations. For fixed panels, longitude has no direct effect on total daily energy, but knowing solar noon helps optimize battery charging schedules and load management.
The longitude correction in minutes equals 4 multiplied by the difference between the standard meridian and your actual longitude. The standard meridian is your UTC offset multiplied by 15. The formula is: LC = 4 × (UTC Offset × 15 − Longitude). A positive result means solar time is ahead of clock time. A negative result means solar time is behind clock time. Add the Equation of Time to get the total solar time correction.
The maximum theoretical longitude correction alone is 4 × 7.5° = 30 minutes, since time zones are typically within ±7.5° of their standard meridian. However, some countries use non-standard time zones that create corrections of 50 minutes or more (such as France and Spain in the CET zone). Adding the Equation of Time of up to ±16.4 minutes means the total time correction can reach over 60 minutes in some locations and dates, making it important for high-accuracy solar calculations.