Equation of Time
You know how your watch is always perfectly on time, but a sundial can look a little off? That “offness” is called the Equation of Time (EoT). It’s the difference between clock time (mean solar time) and sundial time (apparent solar time). This calculator is essential for serious astronomers, sun-worshipers, and anyone building a perfectly accurate sundial.
Equation of Time
Visualize the difference between apparent and mean solar time
What is the Equation of Time?
The Equation of Time shows the difference between apparent solar time (sundial time) and mean solar time (clock time). This difference can be up to about 16 minutes and is caused by two factors: Earth’s elliptical orbit (eccentricity effect) and the tilt of Earth’s axis (obliquity effect).
Result for Selected Date
Annual Equation of Time
Is Your Clock Lying? Calculating the Equation of Time
1. The Simple Setup: Choose Your View
This tool offers two direct ways to check the Earth’s wobbly timing.
- For a Specific Date: Use this for precision.
- Select Date: Pick any date using the calendar input.
- Click Calculate for Date: The tool instantly gives you the minute difference for that single day.
- For the Big Picture: Use this to see the annual trend.
- Click Plot Full Year Graph: This generates a chart that visualizes the EoT over 365 days.
Why This Matters: Your clock assumes the day is always 24 hours long, but the Earth’s speed changes throughout the year. The EoT tells you exactly how far ahead or behind clock noon is compared to actual solar noon.
2. Reading the Single-Day Results
When you calculate for a specific date, you get three simple values:
- Total EoT: This is the key number, showing the final time difference in minutes. For example, +10.00 min means the sundial is 10 minutes ahead of the clock. -5.00 min means the sundial is 5 minutes behind the clock.
- Eccentricity Effect: This part of the difference comes from Earth’s slightly oval orbit (elliptical shape).
- Obliquity Effect: This part comes from Earth’s tilt (the axis angle).
3. Understanding the Full Year Graph
The graph is the real hero here. It plots the daily difference for the whole year.
- Total Equation of Time (Red Line): This is the combined, final result. You will see two main peaks and two main troughs, proving that the time difference shifts dramatically throughout the year.
- Component Lines (Blue and Green): These show how the orbit (eccentricity) and the tilt (obliquity) combine to create the final, messy solar time.
Would you like to calculate the Equation of Time for today?