Tonight's Sky
Plan your observing session
Plan your observing session
Tonight's Sky is a free session-planning tool for amateur astronomers. Enter your location, equipment, and the night you plan to observe, and the app builds a personalised list of deep-sky objects, planets, and solar system bodies that will actually be visible from your site — filtered by sky brightness, aperture, and your chosen experience level.
The app was built and is maintained by Mark Casazza, an amateur astronomer based in west central Ohio. Mark has been observing the night sky for many years from both suburban and dark locations with a 20" Dobsonian and a Celestron C11. He is also an avid astro-photographer using a 5" ED refractor and ZWO camera. His observing interest spans the full deep-sky catalog — from showpiece globular clusters to faint quasars at the edge of what the sky will allow.
The visibility engine behind Tonight's Sky draws on the contrast-threshold research of Roger N. Clark (Visual Astronomy of the Deep Sky, 1990) and Blackwell's (1946) empirical eye-response data, and is continuously calibrated against Mark's own field observations. The goal is simple: every object on your list should actually be visible — no disappointments.
Object data is sourced from Wolfgang Steinicke's NGC/IC catalog, supplemented by specialist catalogs for globular clusters, open clusters, quasars, and more. The site uses historical sky-brightness data, your local horizon, the Moon, and the length of your session.
Tonight's Sky is a personal project, offered free of charge. Feedback, field reports, and bug reports are always welcome via the contact form below.
Check out the Clear Sky Alarm Clock to help you never miss a clear night under the stars.
What we collect. Tonight's Sky does not require an account and does not collect personally identifiable information. Your location, equipment preferences, and session settings are stored in a browser cookie on your own device so the form remembers your choices between visits. That cookie never leaves your browser and is never sent to our servers beyond the normal HTTP request cycle.
If you use the contact form, your name, email address, and message are used solely to respond to your enquiry. They are not stored in a database, shared with third parties, or used for marketing.
Advertising cookies. This site displays advertisements served by Google AdSense. Google and its partners may use cookies to serve ads based on your prior visits to this site and other sites on the internet. Google's use of advertising cookies enables it and its partners to serve personalised ads to you. You can opt out of personalised advertising at any time by visiting Google Ad Settings or aboutads.info.
Third-party services. Tonight's Sky uses the following third-party services which may set their own cookies:
• Google AdSense — advertising (
Google Privacy Policy)
• Google reCAPTCHA — spam protection on the contact form (
Google Privacy Policy)
• Google Fonts — typeface delivery
• Light Pollution Map API — sky brightness data based on your location
Location data. When you use the location picker, your coordinates are sent to a sky-brightness API to look up local light pollution levels. Coordinates are not logged or retained on our servers.
Data retention. We do not operate a user database. Server access logs (standard web-server logs containing IP addresses and page requests) are retained for up to 30 days for security and diagnostic purposes, then deleted.
Your rights. If you have questions about how your data is handled, please contact us.
This policy was last updated March 2026. We are not lawyers — this policy is written in good faith to be clear and honest. If you have specific compliance concerns, please get in touch.
Tonight's Sky is built on the work of many individuals and organisations who have freely shared their data, research, and tools with the astronomical community. Grateful acknowledgement is made to the following:
Minor Planet Center (MPC) — Asteroid and minor planet orbital elements. The MPC is operated by the International Astronomical Union and maintains the authoritative database of small solar system body orbits.
Comet OBServation database (COBS) — Comet orbital elements and observer-derived photometric parameters. COBS is operated by Observatory Črni Vrh and aggregates visual comet observations from amateur astronomers worldwide, providing the most realistic brightness estimates available for planning purposes.
Smithsonian Astrophysical Observatory (SAO) Star Catalog — Double star data sourced via the SAC (Saguaro Astronomy Club) double star compilation, itself drawn from SAO and related astrometric catalogs.
Wolfgang Steinicke's Revised NGC/IC Catalog — The primary source for deep-sky object data. Steinicke's painstaking revision of the original New General Catalogue and Index Catalogue is the most accurate and complete NGC/IC compilation available to amateur astronomers.
General Catalog of Galactic Carbon Stars (CGCS) — Carbon star data from the third edition of the CGCS, providing positions and magnitudes for over 6,800 carbon stars in the Milky Way.
Light Pollution Map — Sky brightness data used to estimate limiting magnitude at your observing site. Coordinates are looked up against the Falchi et al. (2016) world atlas of artificial night sky brightness, accessed via the lightpollutionmap.info API.
Roger N. Clark — The visibility engine at the heart of Tonight's Sky is grounded in Clark's contrast-threshold model from Visual Astronomy of the Deep Sky (Cambridge University Press, 1990), combined with Blackwell's (1946) empirical eye-response data. This research provides the scientific basis for predicting whether a given object will actually be detectable under real observing conditions.
Claude (Anthropic) — Much of the codebase for this version of Tonight's Sky was developed in collaboration with Claude, Anthropic's AI assistant. From orbital mechanics to database architecture, Claude served as a tireless co-developer, debugger, and sounding board throughout the project.
Have a question, spotted a bug, or want to share an observing report? Fill in the form below and it'll land in Mark's inbox.