How to plan a Milky Way session
Confirm the city and evening date, then look for a core-planning interval and its lower-Moon subset. Read the core’s altitude and compass angle as well as the duration. Only after the geometry works should you compare clouds, haze, local lights, an unobstructed view and site access. No weather percentage or new overall score is generated by this calculator.
What “core visible” means here
The reference is the Sagittarius A* direction, approximately J2000 RA 17h45m40s, Dec −29°00′28″. It is a marker for the bright Sagittarius core region, not an optically visible black hole or the entire Milky Way band. Astronomy Engine transforms this reference to the observing date and horizon. Other portions of the Milky Way can remain above the horizon when this marker is not useful.
Why use a 20° altitude and 20-minute screen?
Both are transparent planning preferences, not universal astronomical boundaries. A low core can be blocked by trees, buildings or a thick atmospheric path; a very short overlap is a fragile basis for a long drive. This screen reports Sun ≤ −18° and the core direction ≥ 20° for at least 20 minutes. It must not be read as “impossible below 20°” or a visibility guarantee above it.
Moon phase is not the whole moonlight question
A bright Moon below the horizon affects the chosen session differently from the same phase high in the sky. The extra subset requires the Moon’s geometric center to be at or below −1°, with the same 20-minute minimum. This conservative filter does not model scattering, angular separation, thin crescent exceptions or real terrain. The displayed illuminated fraction is for an explicitly stated instant, not a constant for the whole night.
Worked decisions and limits
Illustrations, not live forecasts: a high core under 90% cloud is not a useful photographic window; a moonless winter night may have no high Sagittarius core at all. A core window after the park gate closes is unusable without another legal arrangement. The calculator excludes clouds, smoke, Bortle class, detailed horizon masks, target framing and access. A photograph’s contrast is not a naked-eye promise.
Frequently asked questions
Can the tool guarantee I will see the Milky Way?
No. It checks geometric overlap, not real sky brightness, weather, vision or photographic exposure.
Does no interval mean no Milky Way at all?
No. The screen targets the Sagittarius core direction above 20°. Other parts of the galactic band may be available.
Why not use the city’s overall score?
A weather score can be high while the core is below the horizon. Position and darkness must be checked separately.
Is a thin crescent automatically a problem?
No. The lower-Moon subset is deliberately conservative. Primary intervals may still be worthwhile with a faint or distant Moon; assess the actual scene.
Can I use it for another season?
Yes, select an evening from 2020–2040. Geometry can be calculated beyond the weather horizon, but that is not a long-range weather forecast.