How it works
Select any Messier or NGC object from the built-in catalog and enter your observing details.
The tool uses astronomy-engine.js — a pure-JavaScript
ephemeris library — to compute the object's altitude and azimuth
for your location and time entirely in the browser: no internet call is made after the page loads.
Each observation is saved to your browser's IndexedDB,
so your log persists across sessions without any account or server.
The catalog tab shows every object with its status (logged / not yet), constellation,
magnitude, and type. The sky chart is a rudimentary horizon view — a reading aid to
confirm you're pointing the right direction. Progress bars update the moment you save a record.
Use Export CSV to back up all observations or import
them into a spreadsheet. The file contains object ID, date/time, latitude, longitude,
altitude, azimuth, telescope, seeing, transparency, limiting magnitude, notes, and sketch
description.
Frequently asked questions
- What is a Messier marathon?
- A Messier marathon is an attempt to observe all 110 Messier objects — galaxies, nebulae, and star clusters catalogued by Charles Messier in the 18th century — in a single night. It is only possible near the spring equinox (late March to early April) when the entire catalog is briefly above the horizon. This log helps you track which objects you have already observed across multiple nights so you can plan what remains.
- How is altitude and azimuth calculated without an internet connection?
- The tool loads astronomy-engine.js from a CDN on first visit. Once the page is cached by your browser it works fully offline. The library contains a high-precision solar-system model (VSOP87 for planets, IAU 2000A for the Moon) and applies accurate precession and atmospheric refraction. For each deep-sky object the right ascension (RA) and declination (Dec) from the catalog are converted to horizontal coordinates using your latitude, longitude, and local sidereal time derived from the entered date and time.
- Will my observations be lost if I clear my browser data?
- Yes — all data is stored in IndexedDB in your browser. Before clearing browser data, use the Export CSV button in the Session History tab to save a backup file. You can keep that CSV as a permanent record even if the browser storage is wiped.
- How do I log NGC objects that aren't in the Messier catalog?
- The catalog tab includes a curated set of prominent NGC objects alongside all 110 Messier numbers. Use the catalog filter to switch between "Messier" and "NGC" views, or search by object name. The NGC subset covers the most popular visual targets such as the Double Cluster (NGC 869/884), Blinking Planetary (NGC 6826), and many showpiece galaxies and clusters.
- What do the seeing and transparency ratings mean?
- Seeing describes atmospheric steadiness: 1 (boiling stars, useless for detail) to 5 (rock-steady diffraction rings). Transparency describes how clear and dark the sky is: 1 (thick cloud or heavy haze) to 5 (exceptionally dark and transparent). Recording both ratings helps you correlate observation quality with conditions when reviewing your history. Limiting magnitude is the faintest star visible to the naked eye at zenith — a useful overall darkness metric (4 in suburban skies, 7+ from a dark site).