Krepusko: Crescent Guide

Krepusko helps you prepare for a young-crescent observation from the place where you are standing.

Before sunset, it calculates when the viewing window opens, how long the moon remains above the horizon, its age, altitude, azimuth, elongation, and crescent width. The astronomy runs on your phone, so the complete personal experience continues without a signal.

In the field, a dark-adapted horizon view points toward the relevant part of the western sky. Face the place where the sun went down and use that known bearing to correct the phone compass. Krepusko then shows the direction; altitude in degrees and fist-widths; the clear horizon you need; a measured uncertainty cone; and a countdown to the useful window.

The detailed view keeps two published visibility methods separate: Yallop (1997) and Odeh (2004). If they differ on a borderline evening, Krepusko shows the disagreement instead of averaging it away. A crescent below the physical Danjon limit remains impossible regardless of the number of reports.

After observing, save one fixed choice:

• Saw it with the unaided eye
• Saw it with binoculars or a telescope
• Looked and did not see it
• Could not see the horizon

Keeping the entry only in your personal record and adding it anonymously to the evening evidence are equal choices. Shared observations have no name, profile, message, or photograph. Your precise location never leaves the phone; it is rounded to a roughly 5 km area before transmission. You can withdraw a shared copy in two taps, even while offline.

The Evening view shows regional counts and sky conditions, never a map or feed of people. Regional content can be reported and hidden. Your private record remains available offline and can be exported as JSON and CSV without an account.

Set a month start date used by your household or community and choose a reversible local, regional, global, or calculation display scope. Krepusko never announces that a month has begun and never asks about a school, sect, or community. It supplies measurements and an observation record; the appropriate scholarly authority makes the decision.

Optional approximate location saves manual setup. A complete manual-place alternative works without location permission. Optional notifications are limited to the viewing window opening soon and the evening evidence being ready.