Mobile planetarium apps have replaced printed star charts for most amateur astronomers. They're accurate enough for pointing a telescope, fast to operate with one hand while the other holds an eyepiece, and some offer features — satellite tracking, ephemeris calculations, ISS pass predictions — that would have required a dedicated computer a decade ago.
The three apps below account for the majority of usage among the amateur community. Each has a distinct design philosophy, and the differences matter in practice.
Stellarium Mobile (free / Stellarium Mobile Plus)
Stellarium began as an open-source desktop planetarium application and remains widely used in that form. The mobile version inherits the same rendering engine, which means the star field simulation is visually accurate to a degree that competitors don't match at the free tier. Star colours, atmospheric refraction near the horizon, and the rendering of deep-sky object shapes are all noticeably better than equivalent apps.
Strengths
The free version includes the full Hipparcos star catalogue (about 120,000 stars), the NGC/IC deep-sky catalogue, and major solar system bodies. The sky simulation can be run forward or backward in time, which helps with planning — checking what Saturn's ring orientation will look like in two weeks, for example, or finding when Jupiter's moons will form a specific configuration.
Stellarium Mobile Plus (a one-time purchase) adds the Gaia DR2 catalogue (~1.7 billion stars), eyepiece simulation showing true field of view, and advanced telescope control via Bluetooth or WiFi (for mounts that support the Meade or Celestron serial protocols). The Plus version also includes DSS (Digitized Sky Survey) photographic overlays, which show what faint nebulae and galaxies actually look like rather than just their catalogue icons.
Limitations
The interface requires some learning. Gestures for navigating the sky, adjusting time, and accessing the settings menu are not immediately intuitive, particularly for users coming from simpler apps. The augmented reality (AR) mode — holding the phone up and seeing a sky overlay — works but is less responsive than SkySafari's equivalent.
SkySafari 6 Pro
SkySafari is developed by Simulation Curriculum and positions itself as a field-use instrument rather than a visualisation tool. The interface is clearly optimised for one-handed dark-adapted use: large buttons, a red-light mode that doesn't disrupt night vision, and fast star identification by pointing the device at any part of the sky.
Strengths
SkySafari's telescope control is the best in class among mobile apps. It supports a broader range of mount protocols than Stellarium Mobile Plus and the connection process is more reliable. For observers with GoTo mounts, the ability to tap an object and have the mount slew to it — without going back to a laptop — is genuinely useful in cold field conditions.
The object database in Pro includes the full Hipparcos catalogue plus the Hubble Guide Star Catalogue (about 15 million stars), the entire NGC/IC, and curated observing lists for visual observation. The "Tonight's Best" feature updates automatically with current dates and coordinates, which is a useful prompt for visitors to new sites who don't have their own observing list prepared.
Offline operation is complete — all data is stored locally, no data connection is needed after initial installation. This matters at mountain locations in Romania where mobile signal is unreliable above treeline.
Limitations
The Pro version costs around €15–20 and does not include the photographic overlays that Stellarium Plus offers. The standard and plus tiers have reduced catalogue sizes. The visual rendering of the sky is less photorealistic than Stellarium — the stars and nebula icons look more like a traditional printed chart, which some users prefer and others find less satisfying.
Star Walk 2
Star Walk 2 (Vito Technology) is the most visually polished of the three and the easiest to learn. It's often the first app downloaded by people with no prior astronomy background, partly because the App Store marketing is effective and partly because the AR star-identification feature works immediately without any configuration.
Strengths
The animation quality — watching planets move through their orbits, seeing the ISS arc across the simulated sky in real time — is better than either Stellarium or SkySafari. For explaining astronomy to children or people encountering it for the first time, Star Walk 2 generates genuine interest quickly. The app also covers satellite passes, aurora forecasts, and meteor shower calendars, which are useful standalone features.
Limitations
Star Walk 2 relies heavily on in-app purchases and a subscription model for deeper features. The star catalogue is smaller than Stellarium's free version. There is no telescope control integration. For anything beyond casual sky identification the app's depth runs out fairly quickly, and the subscription approach means ongoing cost that Stellarium and SkySafari (with a one-time purchase) avoid.
Astrospheric and Meteoblue — for weather, not sky maps
Neither of these is a planetarium app, but both deserve mention in any discussion of field observation tools. Astrospheric provides hourly cloud, seeing, and transparency forecasts formatted specifically for astronomical use — it reads ECMWF and GFS model data and displays it as a 72-hour grid with colour coding that reflects the actual observing impact rather than general cloud cover percentages. Meteoblue's astronomy forecast pages provide similar data with slightly different model emphasis.
Using both together and comparing their forecasts is standard practice among Romanian amateur astronomers when planning trips to mountain sites, where local weather can diverge significantly from lowland forecasts.
Which to use when
For general sky familiarisation and learning constellations: Star Walk 2 (or Stellarium free). For field use with a telescope, particularly with a GoTo mount: SkySafari Pro. For detailed pre-session planning, time simulation, and photographic overlay comparison: Stellarium Mobile Plus. Most active observers end up using two of these in combination — planning with Stellarium's time controls at home, then switching to SkySafari's telescope control at the eyepiece.
A note on compass calibration
All three apps use the phone's magnetometer for the AR overlay alignment. Magnetic declination in Romania is approximately 5.5° east of geographic north as of 2026, which most apps compensate for automatically when GPS is active. The magnetometer can be disrupted by nearby metal (telescope tube, equatorial mount), which causes the AR overlay to drift from the actual sky position. Pointing the AR feature away from the telescope and recalibrating (usually by tracing a figure-8 with the phone) corrects this in under 30 seconds.
Desktop alternative: Stellarium desktop application
The free Stellarium desktop application (Windows, macOS, Linux) is useful for pre-session planning on a large screen. The same catalogue and rendering engine as the mobile app, but without the AR feature. Script automation allows pre-computing object positions for specific dates and times, which is useful for planning targeted observation lists for meteor events or specific conjunction dates.
