About Backyard Orbit

What it is

Backyard Orbit answers the practical questions that come up at the eyepiece: what magnification an eyepiece gives, whether the Moon will fit in the view, which binoculars suit dark skies, how dark your sky really is and when the next good meteor shower falls. Jargon such as exit pupil, true field, f-ratio and Bortle class is explained in plain English, with millimetres and inches side by side.

How it works

Every calculation runs in your browser. The pages need no account and send nothing you type anywhere; once a page has loaded it keeps working offline. The meteor data and object sizes are built into the pages, and the Moon's phase at each meteor peak is computed on your device.

Where the numbers come from

Each page ends with a Sources list naming every formula and data set it uses: the International Meteor Organization's meteor shower calendars, NASA's planetary fact sheets, the SEDS Messier catalogue, John Bortle's 2001 Sky & Telescope article, Jean Meeus's Astronomical Algorithms and others. Where a source could not be found for a figure, it was left out. Rules of thumb such as the maximum useful magnification are labelled as such, and estimates such as the faintest visible star are rough guides: your eyes, your sky and the steadiness of the air all change what you actually see.

Safety

Never point a telescope, binoculars or a finder at the Sun unless a proper solar filter made for that instrument covers its whole front opening. The American Astronomical Society explains how in Solar Filters for Optics. The Sun is deliberately left out of the field-of-view simulator.

Red light mode

The "Red light" button beside the theme switch turns every page red on black, so you can check a number at the telescope without wrecking your night vision. Dark-adapted vision relies on the eye's rods, and at 650 nm, deep red, they are only about a thousandth as sensitive as at their blue-green peak. Screens are not pure 650 nm red and are still bright, so turn your screen brightness right down as well. Your choice is remembered on this device.