CosLog
Every night you observed, written down properly.
CosLog is a diary for astronomers and astrophotographers. Log what you saw and when, from where, through what, in what conditions — then find it again by object, by constellation, or by the night it happened. It draws star charts, exports to PDF, and keeps your photographs alongside the entries they belong to.
It runs entirely in this browser. There is no account, no server holding your observations, and nothing to install.
Your diary lives in this browser — so keep a copy
Nothing you write is uploaded anywhere. It also means the copy in this browser is the only one that exists until you make another. Download a backup at the end of a session you care about. It is one zip file holding the whole diary and every photograph, and it is what you load next time you sit down — on this machine or any other.
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Log your session
Add entries as you observe. Your work is saved in this browser automatically as you go.
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Download a backup
Settings → Backup & restore → Download backup. One zip, yours to keep wherever you keep things.
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Restore it next time
CosLog offers the restore button when it opens with an empty diary. Everything comes back as it was.
What's in it
All of it works offline once the page has loaded — the astronomy is computed here, not fetched from anywhere.
A dashboard you arrange
Counts, tonight's Moon, a gallery of your own photographs, what you logged last and what you have planned next.
Entries with real detail
Object, time and timezone, site, equipment, conditions, notes, and any number of attached photographs.
Catalogues to pick from
Messier, Caldwell, curated NGC and IC highlights and the Solar System, ready to go — with the complete NGC and IC from OpenNGC there too, one click away when you want all 13,000. Add catalogues of your own as well.
Star charts
An interactive chart for any position, and the same chart as vector PDF — one engine behind both, so they always agree.
Satellite transits
Find ISS and other passes across the Sun or Moon from your site, with the geometry worked out for you.
Exports that leave
CSV, JSON, XML, ICS and PDF, for one entry or the whole diary. Your records are never trapped in here.
RA 05h 35m · DEC −05° 23′ · M42 · THE ORION NEBULA
Privacy, by construction
There is no backend to send anything to. The site hands your browser a set of files, and everything after that — the database, the charts, the PDF writer — runs on your own machine. The only time CosLog reaches the internet is to refresh satellite orbital elements, and only if you open that tab.
Support the project
CosLog is free and always will be. If it has been useful to you and you would like to put something in the tip jar, it is much appreciated.
Bugs and feature requests
Found something broken, or want CosLog to do something it doesn't? Get in touch on Facebook. Bug reports are most useful with the browser you were using, what you were doing, and what happened instead.
Terms and credits
Copyright
CosLog and its source code are © 2026 CosmicDroid. All rights reserved.
You are welcome to use CosLog for your own observing, personal or professional, at no cost. You may not redistribute it, rehost it under another name or address, sell or sublicense it, or build a derivative work from it, without written permission. Removing or altering these notices is not permitted.
Your observations are your own. Nothing you record here is claimed by this project, and no rights over your diary, notes or photographs are asserted by using it.
No warranty
CosLog is provided as is, without warranty of any kind. Its calculations are intended for observation planning and record keeping, not for navigation, safety, or any purpose where an error would matter. Always check anything critical against an independent source.
Your diary is stored by your own browser, and keeping backups of it is your responsibility. No liability is accepted for lost data, missed events, or any other loss arising from use of this software.
Privacy
CosLog collects nothing. There are no accounts, no analytics, no cookies for tracking, and no server storing your observations — the diary never leaves your browser.
Credits
Two components require attribution, and get it:
- Deep-sky data from OpenNGC by Mattia Verga, licensed CC BY-SA 4.0.
- satellite.js v4.1.4 by Shashwat Kandadai and UCSC, licensed MIT, propagates satellite orbits.
Both keep their own licences, which this notice does not override.