What is astronomical twilight and why does it matter?

Astronomical twilight is when the sun is between 12 and 18 degrees below the horizon. Once it ends the sky is fully dark with no residual sunlight, which is the window deep-sky astrophotographers need for faint targets.

How accurate are the times?

The NOAA solar position algorithm used here is accurate to within about a minute for dates between 1900 and 2100. It ignores atmospheric refraction variations and elevation above sea level, which can shift real horizon times by a minute or two.

What counts as golden hour?

Golden hour is the period when the sun is between roughly 6 degrees below and 6 degrees above the horizon, giving warm, low-angle light. This tool marks it from the start of civil twilight through sunrise, and again from sunset to the end of civil twilight.

Why do I need longitude if I only care about my time zone?

Solar events depend on your exact longitude, not just the time zone. The tool computes the event in solar time then shifts it to your local clock using the offset you supply, so two cities in the same zone get different sunrise times.

What is the difference between civil, nautical, and astronomical twilight?

They mark how far the sun is below the horizon: civil at 6 degrees, nautical at 12, astronomical at 18. Each darker stage suits different work, from landscape blue hour to navigation to faint-object astrophotography.

Astrophoto Golden Hour & Astronomical Twilight Calculator

Knowing exactly when the light changes is the difference between catching a shot and missing it. This calculator computes sunrise, sunset, golden hour, and all three twilight stages for any location and date using the same NOAA solar position algorithm that powers professional almanacs.

How it works

The sun’s position is found from the date’s Julian century, which gives the geometric mean longitude, the equation of time, and the solar declination. For a target sun altitude angle, the hour angle is:

cos(H) = ( sin(altitude) − sin(lat)·sin(declination) )
         / ( cos(lat)·cos(declination) )

The hour angle H (in degrees) is converted to minutes and applied either side of solar noon. Solar noon itself is 720 − 4·longitude − equationOfTime minutes UTC, which is then shifted to your local clock. Each event uses a different altitude angle: -0.833 degrees for sunrise and sunset (allowing for refraction and the sun’s radius), -6 for civil, -12 for nautical, and -18 for astronomical twilight.

Tips and notes

At high latitudes in summer the sun may never reach -18 degrees, so astronomical twilight never ends and the sky stays partly lit all night. The tool reports No darkness in that case. For the warmest landscape light, shoot during the golden hour window flagged around sunrise and sunset; for deep-sky targets wait until astronomical twilight ends. Remember the offset you enter is your standard or daylight-saving offset from UTC, so add an hour during summer time if your region observes it.