- Read this first
- Entering Data
- Running a sight reduction
- Presenting a Map
- Plotting (NMEA 0183 interface)
- Saving and restoring data
- Working with configurations
- Installation and prerequisites
- Tips for Testing (without a sextant?)
- Troubleshooting
- More Information
- Licenses
The code is open source, and comes with no warranty or liability. Read more in the license description.
This app is a tool for celestial navigation, i.e. finding your location with just a sextant and an accurate watch. This works even if you lose internet connection and/or lose a correct GPS signal. It can be used if you are entering a zone of jammed GPS (which is more and more common today as part of signals warfare) or simply as a backup for GPS.
The app takes care of sight reduction, i.e. converting your sextant and clock readings into your position. This can also be done with analog methods (printed papers, plotting tools) and you are advised to at least get acquainted with classical (analog) methods for celestial navigation.
The sight reduction can be viewed in three different ways:
- The coordinate is presented in the app window.
- You can present a map in the browser of your Android unit.
- You can connect an external device (plotter, GPS unit or similar) and see the coordinate on a map display. This is useful for marine operations and/or when using outdoor handheld devices.
Testing this app can be done with a good sextant (which requires careful preparation and adjustments). You can also test it with "sextant simulators", see below. You must also have access to a precise watch/clock (chronometer).
The app is configured as a full screen app. If your phone has soft buttons (home, back, program list) they may become hidden but you can access them through swiping upwards from the bottom.
You can exit the app using the Exit button at the bottom. (This can be useful if you need to restart the app)
The map and help functions invoke the default web browser. Go back to the Celeste app with the back button.
The app is pre-packaged with astrometric data (nautical almanac) from 2024 to 2030. Entering dates outside this range will result in an error. If you want to do sight reductions for historical or future (before 2024 or beyond 2030) observations you can use the script solution.
The app is simple and designed for use in a noisy environment (typically a boat). Sound effects are used for feedback instead of subtle visual effects. The sounds are loud and you are advised to regulate the volume button accordingly.
When you run the app for the first time you will see messages appear,
explaining briefly the mechanisms and what to expect and do.
You can press the checkbox "Don't show this message again"
to get rid of a particular message.
Note: In order to re-instate all messages you can perform a sight reduction
with zero (0) active sights. Disable all three sights and click
"Perform Sight Reduction!".
The app uses the metric system for all input and presented data, with the exception for distance which is measured in nautical miles (nm). A nautical mile = 1.852 km = 1.151 miles.
DRP (Dead-Reckoning Position) is a rough estimation of your current position.
For a 2-star sight reduction it is used to select the correct
intersection point.
A successful 2-star sight reduction will produce two intersections,
and these can be located far away from each other. The DRP selects the
closest intersection.
For a 3-star sight reduction the DRP serves as a way to shorten the
execution time of finding an accurate result.
Normally you should have at least a rough estimation of your
actual position. Sometimes a grossly incorrect DRP (1000s km off)
may result in a failed sight reduction,
due to a conflicting false intersection (which can be located very far away).
At the top you see fields marked Latitude and Longitude. Enter the latitude and longitude for your DRP in these fields.
The format used is "DD:MM:SS", "DD:MM" or "DD" (degrees, arcminutes, arcseconds) Decimal values can be used. Use negative degrees for southern latitudes or western longitudes. ("59", "-35", "-120:34" and "23:34.2" are valid inputs)
Note: The values of these fields are automatically updated if you execute a successful sight reduction. The newly computed coordinate will be your new DRP.
Your sextant work and the selected DRP might not be very accurate. If you suspect non-accurate input you may increase this threshold value. The normal default is "100" (nm), and this will put on these restrictions:
- The intersections be must located at most 100 nm from each other in order to be included in the calculation.
- For a three sights: The DRP should be within 100 nm from each intersection.
If you are a beginner, or insecure about your readings you may increase this threshold to a higher value, "1000" (nm) or more.
The input form contains input field for three observations (sights).
You need to specify two or three sights. The check box Use this sight can be used to eliminate one sight if you only have two. You can even use just one sight but this will result in a failed sight reduction, but the map view (see below) can be used to present the actual circle of equal altitude and this can assist you in your work.
You have to select the name of the object. Choose the Sun, Moon, one of the planets (venus, mars, jupiter or saturn) or a selected navigational star. You find a list of these stars here.
The name of the entered celestial object isn't case-sensitive.
The altitude is the angle between the object and the horizon, as measured by a sextant (Hs). It is specified in degrees, arcminutes and arcseconds in this format: "DD:MM:SS". You may omit arcseconds and arcminutes, and specifiy just "DD:MM" or "DD". Decimal values may be used, e.g. "23:15.2" or "33:21:5.6".
Check this box if you use an artificial horizon. The altitude value will be divided by 2. (An artificial horizon is a simple mirror, often built using a water or metal (mercury) surface. It can be used whenever you don't have access to a physical horizon).
These specify the time for the observation. Time parameters are based on the ISO 8601 format.
Date is specified as "YYYY-MM-DD"
Time is specified as "HH24:MM:SS" (Hours in 24-hour format. PM/AM not used.)
The last part is a timezone specification. Use "+00:00" or "Z" for GMT time. Use "-HH:MM" for western timezones, and "+HH:MM" for eastern and specify the difference vs GMT.
Specify the known index error of the sextant (in arcminutes).
Select between "UPPER", "CENTRAL" or "LOWER". Specifies the location/limb of your sextant measurement, i.e. if you are measuring towards the center or the lower or upper edge of the Moon or the Sun. Note: This setting has no effect on stars, and very little effect on planets. It is mainly used for the Moon or the Sun.
Specify your elevation above the sea surface (in meters), or more specifically: the elevation of your eye. Note: You can use a non-zero observer elevation only if you are not using an artificial horizon. This setting takes care of the effect of the dip of the horizon.
Specify the temperature (in degrees celsius). This setting affects the effects of atmospheric refraction.
Specify how temperature changes with increasing elevation. Default is "-0.01" (normal atmospheric conditions), which means 1 degree celsius lower temperature for each 100 meters. If you have temperature inversions you may increase this parameter to a positive value ("0.1" for one degree temperature increase per 10 meters). This setting affects the effects of atmospheric refraction.
Specify air pressure (in kPa). Normal air pressure is "101" (kPa). This setting affects the effects of atmospheric refraction.
Press the button "Perform sight reduction!". The calculated position will be presented in the field above the button. If you are using 3 stars you will also see an estimation of the accuracy (in nautical miles) based on the proximity of the three central intersections. If the sight reduction fails you will see an error message (and hear an error sound). The sight reduction does not depend on an active internet or GPS connection.
Press the button "Show map!" to see a map representing the last successful sight reduction. Even for failed sight reductions (and single sights) you can display a map. This map can assist you in troubleshooting your sextant readings.
The map display contains the following:
- Circles of equal altitude for the referenced celestal objects.
- Markers for the intersection of the circles of equal altitude and the GP-s of the referenced celestial objects
- Popup fields with information such as coordinates and azimuths.
- A grid net (arc-minute wide) to be used for precise navigation or correlating to printed charts. (For failed sight reduction the grid is degree-wide and covers the entire Earth.)
The map interface requires a browser and an active internet connection to present full map displays with full detail. Without an internet connection you will see a much coarser map on a continental scale. Use plotting for active navigation and map display without internet connection if you have a plotting device available.
Different browsers behave differently. This is a list of test results for some browsers, when tested on a Samsung Galaxy S23 unit. (Other devices may show different results!) We show support for online (internet active) and offline (no internet). The background tile will not work properly when offline (the base map), but older tiles may be cached. We also see if it is possible to touch the minute grid around the intersection point, since this can be a help for hybrid work with paper charts.
| Browser | Works Online | Works Offline | Supports touch of minute gridlines |
|---|---|---|---|
| MS Edge | YES | YES | YES |
| Samsung Internet | YES | YES | YES |
| Chrome | YES | NO | YES |
| Firefox | YES | YES | NO |
| Opera | NO | NO | NO |
| DuckDuckGo | NO | NO | NO |
Note: For a map with a single sight the circle may not touch your location precisely, if your DRP is inaccurate. This is a result of the need for adjustments to the oblateness of the Earth.
The NMEA-0183 interface is a standard protocol for integration with marine plotters. It was originally designed for wired setup, but the app uses a WiFi-based approach.
The messages sent are GGA messages, containing the position received from the latest successful sight reduction.
If you have access to a marine chart plotter or similar device you can easily use the Celeste app as an information source for the coordinate resolved by sight reduction. Connect the plotting device with Celeste using the following connection parameters.
- Connection type : NMEA 0183
- IP adress : Use the adress you see in the bottom field of the app.
- Port : 10110
- Protocol : TCP
- Direction : Input
If the presented ip address is "No network connection" then you need to connect your phone/tablet to the Wi-Fi where your plotting device is connected.
NOTE: This allows for use (detailed mapping) in scenarios where you lack internet connection!
NOTE: The NMEA server is active for 20 seconds after you have performed a successful sight reduction. You are advised to check for the position update on your plotter, and maybe make an additional marker to save the position.
Connecting through Wi-Fi as described above requires a separate Wi-Fi router. You may also try using a connection based on a Mobile Hotspot. Unfortunately the exact way of doing this varies between vendors and device models, making it difficult to provide exact instructions here. If you use the Celeste device as a Mobile Hotspot server you need to find the "router address" of it and attempt connect to it from your plotter devices. Another option is using a second phone as a Mobile Hotspot server.
The app broadcasts the position only for 20 seconds after the last performed sight reduction. This is for energy conservation.
You can press the "Copy pos" button to save the currently computed position to the clipboard.
You can press the "Copy Data" button to copy the results of the last successful sight reduction to the clipboard. You can use this to save your work easily, using a document solution of your own choice. Just paste the contents into a suitable document in another app, and save.
When you press the button "Paste Data" you can transfer back a saved configuration from another app through the clipboard. If the clipboard does not contain a valid configuration you will hear an error sound, and the paste operation will be aborted.
The app is simple and contains no support for managing your configurations/parameters. But through using the clipboard support (see above) you can easily build your own support using suitable tools (Google Docs may be a solution).
Regarding maps: To save maps you are advised to use the print functionality of your web browser, and use the "print to PDF" option if available.
If you use an external plotter you can use it to build routes, document waypoints etc.
The application runs under Android versions 13-16. A 64-bit ARM processor is required.
There is also an ongoing test program on Google Play. See this discussion.
See this article for info on how to install APK files on Android.
You will need a Web Browser for the mapping function. See the list here for a list of browsers and potential issues. But you will likely have to check and test browsers yourself and choose the best one for your device and configuration.
Celeste is built on the P4A platform (Python for Android) and this is a convenient way of distributing apps coded in Python. The underlying code (and many used libraries) are written in Python and this has motivated this implementation choice.
There are however some implications from this:
-
The memory requirement is about 200 MB. Make sure your phone or tablet has enough memory.
-
On modern phones or tablets the execution speed is good, but you may find the app a litte "sluggish" on older phones/tablets.
You can test the app with or without a sextant. We go through some options, going from simple to more complex.
For simple sextant-free testing: Download the app GPS Anti Spoof or GPS Anti Spoof Pro and use it to collect altitude values. Make sure you use true sextant readings (Hs), take careful notes on limb positions used, and take notes on observed times. Insert the readings into the Celeste app, and you can check your position. Using three sights you should reach 1-2 nautical miles accuracy.
You can also use online star atlases such as Stellarium but this is a little more time-consuming.
For really ambitious sextant-free testing you can use official navigation software such as NOVAS published by the Astronomical Tools department of the US Navy
Another option is testing with a sextant app using the camera, such as CamSextant. Note however that a mobile phone sextant app will never produce readings accurate enough for precise navigation.
You may of course also borrow sights from other navigators, and you can easily find groups on Facebook and other social media where sextant/chronometer readings are shared. Another source is using courses and training material.
And finally you can of course test your own skills with a real sextant.
Do you have problems getting a correct sight reduction? In such case just use two sights, by unselecting "Use this sight" for one of the sights. You can also use just one sight through unselecting the check box for two sights. This will result in an error, but you can watch the map output anyway afterwards. Watch the map carefully to pinpoint any incorrect input. Note: the map function is available also after a failed sight reduction.
More information about testing can be found here.
Entering parameters requires accuracy. Here is a list of common errors
- You forget to specify the correct limb correction.
This will typically result in a circle being about 15 nautical miles off and a bad sight reduction. - Your time zone is off.
This will typically lead to failed sight reductions.
--> Carefully check the timezone - Various typos.
Names of celestial objects, timestamps and angles must be specfied correctly.
You will often get an error message in the coordinate output field.
--> Check the error message and correct your input.
If you use very large fonts, or wide spacing you may encounter problems in the layout of the input form. From android system settings reduce the system font size if these problems affect your work.
The app has been tested (using the NOVAS package) with a large number of various sight reductions, and the expected mean algorithmic error is around 0.2 nm. If you find suspected algorithmic errors larger than 1.0 nm you are advised to check you input carefully. Be careful to not select opposing stars or intersections creating very narrow intersection angles. For more information see the documentation
It has been observed (rarely) that the screen may freeze if you reactivate the app from hidden state. Stop and restart the Celeste app if this happens.
In some rare occasions the app may freeze when you click the "Show map!" button. This happens if your network has degraded to very slow speed, but still with a (barely) live connection/ip-address. If you encounter this problem you are advised to disable wifi and mobile internet manually in Android settings.
In some rare cases the map display will show a coarse map despite you having an active internet connection. The cause is unknown but likely caused by a temporary disturbance of your internet connection. Retry the map generation and check your internet connection.
This app is part of an open-source software library for celestial navigation. You can find more information here.
© August Linnman, 2025, email: august@linnman.net
Celeste Software: MIT License
Licenses for used software libraries and services
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