How to Use Tonight Observing Score
Read tonight's 0-100 score, its Go, Maybe or Skip label and the factor chips behind it, then decide whether the night is worth the trip.
What this tool answers
Is tonight worth going out for at the place you pin? The tool gives one score from 0 to 100 and a label: Go, Maybe or Skip. Cloud carries 40 of the 100 points. Dew risk, haze, astronomical darkness and moonlight share the rest, and rain risk takes points off. How Tonight Score Is Weighted explains every weight.
What you see on the screen
- Location bar: your pin's place name and coordinates, with My location and Change buttons.
- Score card: a round dial with the score and label, a one-line headline and a row of factor chips.
- Under the dial: a Best clear window line and a Best night this week line. During an active meteor shower an Active: (shower name). Simulate radiant link appears too.
- Buttons: Full stargazing weather, Check your Bortle, What's in the sky, Tonight by city and How the score is weighted.
- Score bands: three boxes that explain the labels.
Tonight's live score for Georgetown or Lethem, with its chips, the cloud hour by hour and the week's nights.
Times use a 24-hour clock in the pinned place's local time. For a Guyana pin that is America/Guyana (UTC-4, no daylight saving).
Step by step
- Open Tonight Observing Score.
- Check the place name. Without a saved pin the tool starts from GPS (if you allowed it before), your approximate network location, or "Georgetown (Default)".
- Tap My location for GPS, or Change to pick a town under Guyana Presets.
- For another site, fill Latitude and Longitude under Manual Coordinates and tap Apply Coordinates.
- Wait for "Reading the sky overhead" to give way to the dial.
- Read the score, the label and the chips.
- Check Best clear window for when to be outside.
- On Go or Maybe, tap Full stargazing weather for the hourly forecast at the same pin.
The real tool with a dated forecast for Georgetown: the dial, the factor chips behind it, the best clear window, and what Go, Maybe and Skip mean.
How to read the results
| Score | Label | Text on screen | |---|---|---| | 70-100 | Go | Clear enough to drive for. | | 45-69 | Maybe | Watch dew and gaps. | | 0-44 | Skip | Cloud, dew, moon, or no dark window. |
The headline starts with Go tonight, Maybe tonight or Skip tonight. When something drags the score down, it names the factor costing the most points, such as high dew risk.
| Chip | What it measures | Points | |---|---|---| | N% cloud | Mean cloud over tonight's night hours | Up to 40. Each 10% of cloud costs 4 | | low dew, watch dew, high dew | High: humidity 92% or more, or air temperature within 2 °C of the dew point. Watch: 80% or more, or within 4 °C | 15, 8 or 0 | | transparency excellent, good, fair, poor or hazy | Haze from an aerosol forecast, not telescope seeing | 10, 8, 5, 2 or 0 | | N.Nh astro dark | Hours with the Sun more than 18° below the horizon | 3 per hour, full 15 at 5 hours | | moon N% lit | Moon brightness times the share of the night it is up | Up to 15 | | rain risk N% | Highest rain chance during tonight's astronomical darkness, shown only when it costs points | -8 at 30% or more, -15 at 50% or more | | clear window soon | Best clear window starts within 18 hours | +5 |
A chip ending in n/a means that data did not load, and the factor gets middle points. moon n/a can show for a moment while the Moon calculation finishes.
The tool's score engine with every factor on a slider: start from an example night and see what each change is worth.
Everything else is perfect and the night's mean cloud is 50%. How many of the 40 cloud points does the score keep?
- 40
- 30
- 20 (the right answer)
- 0
Each 10% of cloud costs 4 points, so 50% cloud keeps 20 of the 40.
Best clear window reads like "Best clear window: tonight · 19:00-23:00 · good". It is the best run of night hours with cloud at 30% or less (60% if nothing clearer exists), rain chance under 50%, humidity under 95% and a temperature to dew point gap above 1.5 °C. It searches the 7-day forecast, so it can name a weekday. With no such hours, the card says "No clear, dry viewing window in the next few nights".
Best night this week ranks up to 7 nights by the dial's rules, each with its own Moon and rain chance but without the clear-window bonus. When it names Tonight, it shows the dial's own score.
Using it from Guyana
- Astronomical darkness lasts about 9 to 10 hours a night in Georgetown and Lethem all year, so the dark factor almost always earns its full 15. Cloud, dew, the Moon and rain decide the score.
- The coast is wet from mid-April to the end of July and mid-November to the end of January. Rain is more likely then, so check again in the late afternoon.
- Pin the site you will stand at. For a Rupununi trip, pick Lethem, not your home town.
- If the chip reads watch dew or high dew, read the Dew and Monsoon Stargazing Playbook.
What it does not do
- It ignores light pollution. Check your Bortle opens the Bortle Map.
- It scores tonight only, with no date picker and no hourly chart. Stargazing Weather has the hours.
- It gives no Moon or twilight clock times. Use Sun/Moon Times & Dark Window.
- It says nothing about the ISS or planets. Use the ISS Pass Predictor and Planets Visible Tonight.
- It cannot see a small shower or local fog that the forecast misses.
Common mistakes
- Scoring your home while you plan to drive to a darker site.
- Reading Go as a Milky Way promise. The score counts the Moon, not light pollution.
- Treating a later night's score as settled. Its forecast will change, so check again on the day.
- Checking once at noon. Cloud can build or clear within hours, so check again before you leave.
The dial reads 82 Go for your Georgetown backyard. Does that promise the Milky Way?
- Yes: 82 is a Go
- No: the score ignores light pollution, so check the Bortle class too (the right answer)
- Only if the Moon chip is missing
- Only after midnight
The score weighs cloud, dew, haze, darkness, the Moon and rain, but not light pollution: central Georgetown is about Bortle 8, where the Milky Way does not show.