Dark Sky Planner

Selecting a location

Search by city, town, or ZIP code to find your observing site — results come from OpenStreetMap Nominatim. If your site is in a remote area without a searchable name, use the map picker to drop a pin at exact coordinates. You can also click to use your device's GPS position if you are already at your observing site.

Once a location is selected, location name, timezone, latitude, and longitude are filled in automatically. All four fields can also be entered or adjusted manually — the search and map methods are simply faster. The location name appears on the report but does not affect calculations; latitude, longitude, and timezone drive all astronomical computations.

If you're planning a trip, verify the timezone matches your destination — all window times and dates are interpreted in the site's local timezone.

Setting your observing window

The window defines the time span evaluated each night. The start can be a solar event — Sunset, Civil Dusk (Sun 6° below the horizon), Nautical Dusk (Sun 12° below), or Astronomical Dusk (Sun 18° below) — or a fixed clock time. The end is set the same way, using Astronomical Dawn, Nautical Dawn, Civil Dawn, Sunrise, or a fixed time.

The planner evaluates Moon conditions only within your window. A Moon that rises after your window ends, or sets before it begins, has no effect on the rating.

At high latitudes, the Sun may not dip far enough below the horizon for twilight events like Astronomical Dusk to occur on every date in your range — particularly in summer months. When that happens, nights are excluded because the window boundary cannot be established. If you are observing at a high latitude, consider using a fixed time for your window start and end to avoid this.

Moon rules

Moon below horizon or under illumination threshold — a night qualifies if the Moon is below the horizon for the full window, or if it is above the horizon but its peak illumination during the window stays at or below your threshold. This allows some nights when a low-illumination Moon is technically above the horizon.

Moon below horizon only — a night qualifies only if the Moon is entirely below the horizon for the full window. Any moonrise during the window causes the night to be excluded.

The illumination threshold only applies to the first rule. A lower value is more restrictive — fewer nights qualify. A higher value is more permissive — more nights qualify, including some with a partially lit Moon above the horizon. A threshold of 10–20% suits most visual observers; astrophotographers targeting faint objects often want stricter limits.

Tips

• Astronomical Dusk marks when the Sun is 18° below the horizon — the standard definition of astronomical night. If you want to ensure full darkness before your window begins, this is the appropriate window start.
• Use the day filter to focus the report on nights you can actually use. Filtering to Friday and Saturday, for example, removes weeknight results that don't fit your schedule.
• Use the map picker when your observing site doesn't have a searchable name, or when you want to enter precise coordinates rather than a city center.
• Use it for visual astronomy, telescope observing, dark sky trips, and astrophotography planning.
• Enable Show excluded dates on the report to see each excluded night alongside the reason it didn't qualify — useful when you expect a date to appear but it doesn't.
• The report URL encodes all your inputs — bookmark it or share it to revisit the exact same plan.