How to Use Sun/Moon Times & Dark Window

How to Use Sun/Moon Times & Dark Window

Pick a date and read the longest stretch of that night with the Sun below -18° and the Moon below the horizon.

Open Sun/Moon Times & Dark Window

All times in this guide are Guyana time (America/Guyana, UTC-4, no daylight saving), written the way the tool shows them.

What this tool answers

For one place and one night: when do the Sun and Moon rise and set, and what is the longest moonless stretch of astronomical night? That stretch is your best time for faint deep-sky targets and the Milky Way. Is Tonight Good for Stargazing? explains why darkness and the Moon matter, and The Moon and Lunar Phenomena explains the phases.

What you see on the screen

The page heading reads Sun, Moon & Dark Window.

• Location bar: place and coordinates, with My location and Change.
• Left column: an Observer panel with Use my current location and a note naming the time zone, then a Date (YYYY-MM-DD) picker, a Use today link, a Compute button, and See also links.
• Sun card: Sunrise, Sunset, Solar noon (time and the Sun's height), Day length; then a Twilight card: golden and blue hour, civil (-6°), nautical (-12°) and astronomical (-18°) dusk, and the same at dawn.
• Moon card: Moonrise, Moonset, Highest (time and altitude), Phase (name and % lit), Brightness · size; then Planets highest tonight, each naked-eye planet's time and altitude at its highest.
• Dark Sky Window card: a large duration such as 9h 41m, the line "Moonless astronomical darkness (sun below -18°)", Starts and Ends, then a Timezone line.
• Add to calendar button under the card, shown only when there is a window.

On narrow screens the left column sits above the results.

Step by step

1. Open Sun/Moon Times & Dark Window.
2. Check the place. Tap My location for GPS, or Change to pick a Guyana Presets town or enter Manual Coordinates.
3. Pick the date of the evening you plan to go out. The results update as soon as you pick a full date; Compute applies a date you typed.
4. Read the Dark Sky Window length, then Starts and Ends.
5. Check Moonrise, Moonset and Phase to see why the window is long or short.
6. Tap Add to calendar to download the window as a calendar file.
7. Change the date and repeat to compare nights.
8. Open Stargazing weather under See also to check clouds for the same place.

How to read the results

• The date means the night that starts that evening. Sunset is that evening and Sunrise is the next morning, when the night ends. Day length is the daylight on the chosen date.
• Moonrise and Moonset belong to the Moon's pass that matters for that night. If the Moon is already up at sunset, Moonrise can be earlier than Sunset, and Moonset can fall after midnight or after sunrise. A dash means no time was found.
• Phase gives the phase name and the percent lit at the middle of the night, halfway from sunset to sunrise.
• Dark Sky Window is the longest run inside astronomical night (Sun more than 18° below the horizon) with the Moon at or below the horizon. Starts and Ends use the exact moonrise and moonset, so when the Moon rises or sets during astronomical night they match those times.
• A thin crescent still above the horizon breaks the window, even when it barely brightens the sky.
• On nights near full Moon you get "No moonless astronomical darkness for this night" instead of a duration.
• For a pin outside Guyana, times switch to that place's own time zone, named in the Observer note and the Timezone line.

Any night of the coming months on the ribbon, with the tool's own Sun, Moon and Dark Sky Window cards and the window drawn in gold.

Astronomical night begins at 18:50, but a thin crescent Moon does not set until 19:40. When does the Dark Sky Window start?

• 18:50, when astronomical night begins
• 19:40, when the Moon sets (the right answer)
• At sunset
• There is no window while any Moon is up that night

The window needs the Sun more than 18° down and the Moon at or below the horizon, so even a thin crescent holds it back until moonset.

Georgetown on two nights of October 2026: a new Moon gives a moonless window of 9h 41m, from 6:49 PM to 4:30 AM; the Full Moon of 25 October leaves none.

Using it from Guyana

Twilight is short near the equator. From Georgetown the Sun reaches -18° about 69 to 77 minutes after sunset through the year, and astronomical night lasts about 8.9 to 9.8 hours. Day length only runs from about 11.7 to 12.5 hours; Why Seasons Happen explains why. So a moonless night gives a window of roughly nine to ten hours, and the Moon decides the rest.

How long twilight lasts and how long astronomical night runs, through the year, at Georgetown and Lethem against London.

What the tool shows for Georgetown in October 2026:

| Date picked | Phase shown | Dark Sky Window | |---|---|---| | 2026-10-03 | Last Quarter · 43% lit | 5h 21m, 6:52 PM to 12:13 AM (Moonrise 12:13 AM) | | 2026-10-10 | New Moon · 0% lit | 9h 41m, 6:49 PM to 4:30 AM | | 2026-10-18 | First Quarter · 55% lit | 4h 23m, 12:06 AM to 4:29 AM (Moonset 12:06 AM) | | 2026-10-25 | Full Moon · 100% lit | No window |

Around last quarter the dark comes in the evening; around first quarter it comes after moonset, late in the night. Lethem (3.4°N) gets almost the same: 9h 38m on 2026-10-10, from 6:57 PM to 4:35 AM.

What it does not do

• It works one date at a time; there is no calendar view. For a week, use the 7-night outlook in Stargazing Weather or Best night this week in Tonight Observing Score.
• It does not include clouds, rain or dew.
• It gives the Moon's highest point, not its height through the night or how much a crescent brightens the sky. The Moonlight Impact Calculator shows the Moon's altitude and % lit right now.
• It does not shorten the window for a faint or low Moon. Starts equals Astronomical dusk only when the Moon is already down.
• It assumes a flat, sea-level horizon, so it cannot tell you when a low Moon drops behind your local trees or hills.
• It ignores light pollution: check your site on the Bortle Map.

Common mistakes

• Picking today's date after midnight. At 1:00 AM, Use today shows the coming evening, not the night you are in. Pick yesterday's date.
• Planning by "new Moon weekend" alone. Check Moonrise and Moonset: on 2026-10-03 a Moon 43% lit still left over five dark evening hours.
• Arriving at sunset and leaving early. In Guyana the window cannot start until about 70 minutes after sunset, and on a moonless night it runs until about 70 minutes before sunrise.
• Reading Starts and Ends as exact to the minute.
• Forgetting the weather. A perfect window under cloud is still cloud.

It is 1:00 AM on Saturday 10 October, and you want the night you are standing in. Which date do you pick?

• 2026-10-09, the evening the night began (the right answer)
• 2026-10-10, today's date
• 2026-10-11, the next evening
• Any: the tool shows the same night

The date means the night that starts that evening, so after midnight you pick yesterday's date; Use today would show the coming evening.

Related

• Concept lessons: Is Tonight Good for Stargazing?, The Moon and Lunar Phenomena, Why Seasons Happen
• Tools: Stargazing Weather, Tonight Observing Score, Bortle Map