Solar Longitude Calculator
Find Solar Time & Time Correction by Location
Solar Longitude Calculator
How to Use Solar Longitude Calculator
Get precise solar time corrections and LST conversions in just a few quick steps:
- 1Enter 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.
- 2Enter 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.
- 3Select 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.
- 4Enter 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.
- 5Click 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.
- 6Read 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
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
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 Tilt, Azimuth, and Seasonal Sizing for Solar Longitude
For maximizing the seasonal or annual output of a solar PV array running Solar Longitude calculations, panel orientation and tilt angle must be carefully optimized. The optimal tilt angle is primarily determined by your geographic latitude, while the azimuth determines the direction the panels face (South in the Northern Hemisphere, North in the Southern Hemisphere):
For fixed-tilt Solar Longitude systems, setting the tilt equal to the local latitude is generally the best year-round compromise. In locations with higher cloud cover during winter, bias the angle slightly toward summer parameters to maximize performance during peak generation months.
Mono vs. Poly vs. Thin-Film Options for Solar Longitude
Choosing the correct cell technology determines the efficiency and spatial footprint of your Solar Longitude installation. Monocrystalline panels offer the highest efficiency (20%+), followed by polycrystalline (15-18%) and thin-film (10-13%):
| Technology | Typical Efficiency | Temperature Tolerance | Space Required |
|---|---|---|---|
| Monocrystalline | 20% - 22% | Excellent (-0.37%/°C) | Minimal |
| Polycrystalline | 17% - 19% | Moderate (-0.41%/°C) | Moderate |
| Thin-Film (Amorphous) | 11% - 13% | Superb (-0.20%/°C) | High |
Monocrystalline panels are highly recommended when roof space is constrained, whereas thin-film is suited for flexible surfaces or hot climates due to its superior temperature coefficient.
Frequently Asked Questions (FAQs)
Solar longitude is an angular measurement indicating the Earth's exact position in its yearly orbit around the Sun. Understanding this metric is vital for complex astronomical calculations, designing efficient tracking solar arrays, and accurately predicting seasonal shifts in daily available.
Geographic longitude measures your exact east-west position on the Earth's surface relative to the Prime Meridian. In direct contrast, solar longitude measures the Earth's orbital progression around the Sun, marking specific seasonal events like equinoxes and solstices throughout the calendar year.
Calculating solar longitude requires complex astronomical formulas factoring in the Julian date, time, and orbital mechanics. Because manual calculations are highly tedious and error-prone, utilizing an online calculator ensures instant and exceptionally accurate orbital results for any specific.
Yes, solar longitude directly dictates the angle of sunlight striking your panels across different seasons. By predicting these precise orbital changes, commercial solar farms can intelligently program automated tracking systems to flawlessly follow the sun and significantly maximize energy.
By astronomical definition, solar longitude is exactly zero degrees at the moment of the vernal equinox, marking the official start of spring in the Northern Hemisphere. It progressively increases to ninety degrees at the summer solstice and consistently grows throughout the entire calendar year.