Astronomical Tools

Interactive Sky Chart

Find out what's in your sky tonight — create a custom map of the night sky for your location!

Astronomical Tools

Astronomical Almanac

When do the Sun and Moon rise and set? When does twilight end and begin? Which planets are up? Start your night of observing with our Astronomical Almanac.


Find the Phase of the Moon

Learn the phase of the Moon tonight, the day you were born, or on any historical date.

Celestial Objects to Observe

Find Jupiter's Moons - Interactive Observing Tool

Use our interactive observing tool to say which of the planet's four largest moons is which.

Celestial Objects to Observe

Transit Times of Jupiter's Great Red Spot

Calculate when the Great Red Spot will cross Jupiter's central meridian — that's the best time to see the famous storm through your telescope.

Celestial Objects to Observe

Find Saturn's Moons: Interactive Observing Tool and App

With help from our interactive JavaScript utility, you can always tell which of Saturn's brightest moons is which.


The Elusive Moons of Uranus

Take the observing challenge: Find as many as five of the brightest moons of Uranus in a large backyard telescope using our interactive observing tool.

Celestial Objects to Observe

Track Triton, Neptune's Largest Moon

Triton, Neptune's largest moon, is a tricky find. Our Triton Tracker observing tool can help users of moderate to large telescopes spot this distant moon.

Astronomical Tools

Telescope Calculator: How Does Your Telescope Perform?

Use this telescope calculator to tell you how changing out eyepieces and accessories will affect your telescope's performance.

Celestial Objects to Observe

Mars Profiler: Which Side Is Visible?

To compare what you see on Mars with a map, you need to know which side of the planet you're looking at. Our handy Mars Profiler tells you that and more, for any date and time.

Celestial Objects to Observe

Satellite Tracking Tool: Track the ISS & Hubble

The International Space Station passes over virtually all of Earth's populated areas, and you can spot it easily with your eyes alone — if you know where and when to look for it.

Astronomical Tools

Satellite Transit Tool: Spot ISS Transits of the Sun and Moon

The International Space Station often passes close to the Moon, Sun, and naked-eye planets. Use this tool to plan viewing these close encounters.

Variable Stars

The Minima of Algol

Now you can calculate the dates and times (local and Universal Times) when the eclipsing variable star Algol should be at its dimmest (magnitude 3.4 instead of its usual 2.1).