Sun Time Calculator: Find Solar Time

A sun time calculator works out the local solar time at your location, which is the time based on the Sun’s actual position in the sky rather than the clock time on your wall. You enter your longitude, the current clock time, and your time zone offset, and the calculator returns the solar time, the difference from clock time, and often the times of sunrise and sunset for that day. This is the time used for gardening, solar panel orientation, photography, and any activity where the Sun’s position matters more than what your watch says.

Formula

The basic formula for solar time is:

Solar Time = 12:00 + (Longitude / 15) − Time Zone Offset + Equation of Time

Where:

• Longitude (in degrees, west negative) – your geographic longitude.
• Time Zone Offset (in hours) – the difference between your clock time and UTC (e.g., Eastern Standard Time is −5).
• Equation of Time (in hours) – a correction that accounts for the Earth’s elliptical orbit and axial tilt. A common approximation is:

EoT = 0.170 sin(4π(d − 80) / 373) − 0.129 sin(2π(d − 8) / 355)

where d is the day of the year (1–365). This gives the equation of time in hours, accurate to within about 30 seconds for most of the year.

The calculator above uses your clock time (in 24-hour format) and adds the longitude correction and the equation of time to get the solar time. The difference output is simply the solar time minus the clock time, expressed in minutes.

Worked Example

Let’s use the default values: longitude −105° (Denver, Colorado), clock time 14:30 (2:30 PM), time zone offset −6 (Mountain Daylight Time), and day of year 200 (July 19).

1. Longitude correction: −105 / 15 = −7 hours. This means solar noon (when the Sun is highest) occurs at 12:00 − (−7) = 19:00 UTC, but we’ll keep it in local terms.
2. Time zone offset: −6 hours. So the base time is 14:30 + (−7) − (−6) = 14:30 − 1 = 13:30.
3. Equation of time for day 200: 0.170 sin(4π(200−80)/373) − 0.129 sin(2π(200−8)/355) ≈ 0.170 sin(4.04) − 0.129 sin(3.40) ≈ 0.170(−0.782) − 0.129(−0.255) ≈ −0.133 + 0.033 = −0.100 hours = −6 minutes.
4. Solar time = 13:30 − 6 minutes = 13:24 (1:24 PM).

The calculator’s first output shows 13:24, and the difference is −66 minutes (solar time is 66 minutes behind clock time). That matches what you’d expect in July in Denver: the Sun is about 1 hour behind clock time due to Daylight Saving Time and the longitude offset.

Why Solar Time Matters

Clock time is a social construct. It was standardized in the 19th century to make train schedules and communication predictable, but it deliberately ignores the Sun’s position. Solar time, also called apparent solar time, is what a sundial reads. It varies by longitude – every 15 degrees of longitude shifts solar time by one hour – and it also shifts slightly throughout the year because the Earth’s orbit is not a perfect circle and its axis is tilted.

For most daily life, the difference between solar and clock time is small (usually within 30 minutes). But for certain activities, it matters:

• Solar panel orientation – panels should face true south (or north in the southern hemisphere) at solar noon, not clock noon.
• Gardening – knowing when the Sun is directly overhead helps determine when shadows are shortest, which is useful for planting shade-sensitive plants.
• Photography – the golden hour is defined by solar time, not clock time. A solar time calculator helps you plan shoots.
• Astronomy and navigation – celestial navigation uses solar time to determine longitude.
• Sundial design – if you’re building a sundial, you need to know the difference between solar and clock time to calibrate it.

How to Use the Calculator

To get the most accurate result, you need four pieces of information:

• Your longitude – you can find this from GPS, Google Maps, or a gazetteer. West longitude is negative, east is positive.
• Your current clock time – in 24-hour format. If your clock shows 2:30 PM, enter 14:30.
• Your time zone offset – the number of hours you add to UTC to get your local time. For example, Eastern Standard Time is −5, but Eastern Daylight Time is −4. You must use the offset that is currently in effect, including Daylight Saving Time.
• The day of the year – January 1 is 1, December 31 is 365 (or 366 in a leap year). You can find this from a calendar or an online day-of-year converter.

The calculator then outputs the solar time in 24-hour format and the difference in minutes. A positive difference means solar time is ahead of clock time; a negative difference means it’s behind.

Common Mistakes and Limitations

The most common mistake is using the wrong time zone offset. Daylight Saving Time changes the offset by one hour, and many people forget to adjust. For example, in New York, the offset is −5 in winter and −4 in summer. If you use the wrong one, your solar time will be off by an hour.

Another mistake is confusing longitude with latitude. Longitude is east-west; latitude is north-south. Solar time depends only on longitude, not latitude (though the Sun’s height at noon does depend on latitude).

The equation of time formula used here is an approximation. It’s accurate to within about 30 seconds for most of the year, but it can be off by up to a minute near the extremes (early November and mid-February). For precise work, you’d use a more complex formula or an almanac table.

Also note that the calculator assumes you’re using standard time (no Daylight Saving) in the time zone offset. If you enter a clock time that includes DST, you must use the DST offset. The calculator doesn’t know your local DST rules.

Solar Time vs. Clock Time: Real-World Examples

To see how this works in practice, consider two cities at the same latitude but different longitudes:

• New York (longitude −74°) – Solar noon occurs at about 11:55 AM EST in late October, but at 12:55 PM EDT in July. The difference is due to both the longitude and the equation of time.
• Los Angeles (longitude −118°) – Solar noon is around 12:00 PST in winter, but 1:00 PM PDT in summer. Because LA is far west of the center of its time zone, solar time is often behind clock time.
• Beijing (longitude 116°) – Solar noon is about 12:14 PM CST (China Standard Time, UTC+8) because the country uses a single time zone for its entire width, so western parts of China have solar noon as late as 3 PM.

These examples show that the difference can be more than an hour in extreme cases, which is why a calculator is useful.

Frequently Asked Questions

What is the difference between solar time and clock time?

Solar time is based on the Sun’s position in the sky. When the Sun is exactly due south (or north in the southern hemisphere), it’s solar noon. Clock time is a standardized time zone time that doesn’t vary with longitude within the zone. The difference is the sum of the longitude correction and the equation of time.

Does latitude affect solar time?

No, latitude does not affect solar time. Solar time depends only on longitude and the equation of time. Latitude affects the Sun’s altitude and the length of daylight, but not the moment of solar noon.

Why is solar noon not at 12:00?

Because time zones are wide – up to 15 degrees of longitude – and because of the equation of time. If you’re at the center of your time zone and it’s a day when the equation of time is zero, solar noon is at 12:00. Otherwise, it can be up to 30 minutes earlier or later, and more in extreme cases like China.

How do I convert solar time to clock time?

Use the same formula in reverse: Clock time = Solar time − (Longitude / 15) + Time Zone Offset − Equation of Time. Or simply use the difference output from the calculator and subtract it from the solar time.

Does Daylight Saving Time affect solar time?

No, solar time is independent of DST. But the difference between solar and clock time changes by one hour when you switch to DST because the clock time shifts. So you must use the correct time zone offset for the season.

Can I use this calculator for any date in the past or future?

Yes, the formula works for any day of the year, but the equation of time approximation is only valid for the current era (roughly 1900–2100). For very distant dates, the Earth’s orbital parameters change slightly, but the effect is negligible for most purposes.

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in lon | Longitude (west negative) | ° | -105 | -180 | 180
in clock | Clock time (24h) | hh:mm | 14:30 | 0 | 24
in tz | Time zone offset from UTC | h | -6 | -12 | 14
in day | Day of year | - | 200 | 1 | 365
out solar | Solar time | hh:mm | 12 + (lon / 15) - tz + (0.170 * sin(4 * pi * (day - 80) / 373) - 0.129 * sin(2 * pi * (day - 8) / 355)) | time
out diff | Difference from clock | min | (solar - clock) * 60 | 1