About skycrate

skycrate is for the question you ask before going outside at night: is it worth it, and where do I look? It puts tonight's sky, the weather and the space weather for your location on one page, and keeps it current while the page is open.

Where the numbers come from

• Sun, Moon and planets. Worked out directly with Don Cross's Astronomy Engine, which is good to about an arcsecond. That covers rise and set times, twilight, the sky map and the events calendar. The calendar runs in your browser.
• Stars and constellations. About 600 of the brightest stars and the main constellation figures on the sky map, from Olaf Frohn's d3-celestial (© 2015 Olaf Frohn, BSD licence), based on ESA's Hipparcos catalogue.
• The ISS. Orbital elements from CelesTrak. A pass only makes the list if the station is in sunlight while your sky is dark, because otherwise you can't see it.
• Aurora. NOAA's Kp index and OVATION model, compared against how far you are from the magnetic pole.
• Weather. Hourly cloud cover (low, mid and high cloud separately) and humidity from Open-Meteo, which also powers the place search.
• Seeing and transparency. How steady and how clear the air is tonight, from 7Timer!. This matters most if you use a telescope. If 7Timer! doesn't answer, the rows are left out.
• Meteor showers. Dates and rates from the International Meteor Organization's calendar.
• Picture of the day. NASA's APOD.
• News. Feeds from NASA, ESA, Space.com, EarthSky, SpaceNews and a few others, checked about every hour and shown as published.
• Tonight's verdict. A short read written by Google's Gemini from the figures already on the page. It runs when you ask for it, and once automatically if your location was detected for you.

The visibility score

It starts at 100. Subtract the average cloud cover between sunset and sunrise, counting thin high cloud at 60% because stars still show through it, dimmed. If the Moon is up, subtract a quarter of how much of it is lit. If humidity goes over 90%, subtract another 10, because that usually means haze. That's the whole formula. It tells you whether a night is worth planning around, not what you'll actually see.

How much to trust it

The astronomy is exact. Planet positions, eclipse times and moonrise won't be wrong. The weather is the weak part. Cloud forecasts hold up for a day or two and get rough after that. Aurora is harder still, because useful forecasts only reach about an hour ahead, so that card gives odds rather than a promise. If it looks clear outside, go and look.

Privacy

There are no accounts. Your location and saved places stay in your browser. The server only keeps anything if you sign up for alerts, and then only what those alerts need. Full details.

From toolcrate

skycrate is a sister site of toolcrate. A few of its tools are useful here:

• World Clock, for when you're watching the same event as someone in another city.
• Timezone Difference Calculator, for turning an eclipse time into someone else's local time.
• Coordinate Converter, for switching between decimal degrees and degrees-minutes-seconds.