{primary_keyword}
Calculate civil twilight start and end times for any date and location.
Enter Details
Intermediate Values
| Day of Year | Solar Declination (°) | Hour Angle (°) | Civil Twilight Duration (h) |
|---|---|---|---|
| — | |||
Civil Twilight Chart
What is {primary_keyword}?
{primary_keyword} is a tool that calculates the times of civil twilight for any given date and geographic location. Civil twilight occurs when the sun is between 0° and -6° below the horizon, providing enough natural light for most outdoor activities without artificial illumination. This {primary_keyword} is useful for photographers, astronomers, outdoor planners, and anyone needing precise lighting information.
Anyone who works with daylight—such as event organizers, pilots, or hikers—can benefit from a reliable {primary_keyword}. Common misconceptions include believing that civil twilight is the same as nautical or astronomical twilight; in reality, each twilight phase is defined by a different solar depression angle.
{primary_keyword} Formula and Mathematical Explanation
The core of the {primary_keyword} relies on solar geometry. The steps are:
- Calculate the day of the year (N) from the selected date.
- Determine the solar declination (δ) using: δ = 23.44° × sin(360° × (N + 284)/365).
- Compute the hour angle (ω) for the civil twilight altitude (h = –6°) with: cos ω = (sin h – sin φ × sin δ) / (cos φ × cos δ), where φ is latitude.
- Find the civil twilight start and end times around solar noon: Solar Noon = 12 – (lon/15) + TZ, then Twilight Start = Solar Noon – ω/15, Twilight End = Solar Noon + ω/15.
All angles are in degrees, and time is expressed in hours.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Day of year | integer | 1‑365 |
| δ | Solar declination | degrees | ‑23.44 to +23.44 |
| φ | Latitude | degrees | ‑90 to +90 |
| lon | Longitude | degrees | ‑180 to +180 |
| h | Sun altitude for civil twilight | degrees | ‑6 |
| ω | Hour angle at civil twilight | degrees | 0‑180 |
| TZ | Timezone offset from UTC | hours | ‑12 to +14 |
Practical Examples (Real-World Use Cases)
Example 1: New York City, March 21
Inputs: Date = 2024‑03‑21, Latitude = 40.71°, Longitude = ‑74.01°, Timezone = ‑5.
Result: Civil Twilight Start ≈ 06:12 AM, Civil Twilight End ≈ 07:45 PM. The duration of civil twilight is about 1.55 hours each side of sunrise/sunset.
Example 2: Reykjavik, June 15
Inputs: Date = 2024‑06‑15, Latitude = 64.13°, Longitude = ‑21.90°, Timezone = 0.
Result: Civil Twilight Start ≈ 02:45 AM, Civil Twilight End ≈ 11:55 PM. Near the Arctic Circle, civil twilight lasts for most of the day, providing continuous low‑level illumination.
How to Use This {primary_keyword} Calculator
- Enter the desired date, latitude, longitude, and timezone offset.
- The calculator updates automatically, showing the civil twilight start and end times.
- Review the intermediate values table for declination, hour angle, and twilight duration.
- Use the chart to visualize sun altitude throughout the day.
- Click “Copy Results” to copy all key data for reports or planning.
Key Factors That Affect {primary_keyword} Results
- Latitude – Determines the sun’s path and the length of twilight.
- Day of Year – Affects solar declination, changing sunrise and sunset times.
- Longitude – Shifts solar noon relative to the local clock.
- Timezone Offset – Aligns solar times with civil time zones.
- Atmospheric Refraction – Slightly extends visible twilight, not accounted for in the basic formula.
- Elevation – Higher elevations experience slightly earlier sunrise and later sunset.
Frequently Asked Questions (FAQ)
- What is the difference between civil, nautical, and astronomical twilight?
- Civil twilight occurs when the sun is 0° to –6° below the horizon, nautical twilight from –6° to –12°, and astronomical twilight from –12° to –18°.
- Can I use this {primary_keyword} for locations near the poles?
- Yes, but during polar day or night the sun may never reach –6°, resulting in continuous daylight or darkness.
- Why does the calculator sometimes show “NaN”?
- This occurs when inputs are out of range (e.g., latitude beyond ±90°). Correct the values to see valid results.
- Do I need to account for daylight saving time?
- Adjust the timezone offset accordingly; the calculator uses standard offset only.
- Is atmospheric refraction included?
- No, the basic {primary_keyword} ignores refraction, which can add a few minutes to twilight.
- How accurate is the {primary_keyword}?
- For most practical purposes, the results are accurate within a few minutes.
- Can I use this for marine navigation?
- Marine users typically rely on nautical twilight; however, civil twilight information can still be useful.
- Is the chart interactive?
- The chart updates automatically when inputs change, showing sun altitude and the –6° twilight line.
Related Tools and Internal Resources
- {related_keywords} – Sunrise Calculator: Compute sunrise and sunset times.
- {related_keywords} – Moon Phase Tracker: Determine moon illumination for any date.
- {related_keywords} – Solar Position API: Access detailed solar geometry data.
- {related_keywords} – Day Length Calculator: Find total daylight duration.
- {related_keywords} – Atmospheric Refraction Guide: Learn how refraction affects twilight.
- {related_keywords} – Timezone Converter: Convert times between global zones.