Star Charting Apps for iPhone and iPod Touch

Review by Dan Schroeder, Physics Department, Weber State University
(Opinions expressed here are mine, not those of WSU.)

Update badly needed! This review was last updated in early July 2009, but even then I didn't have time to bring it fully up to date. Now it has fallen even farther behind the times, as three new developers have introduced star-charting apps and several of the older apps have gotten major upgrades. Here's a summary of the new developments as of 21 December 2009:

Owners of the Apple iPhone and iPod Touch are fortunate to have a great selection of low-cost astronomy applications, available from the iTunes Store. Among them are no fewer than seven serious star-charting apps that will show you the constellations, planets, and much more. Each of these apps has its strengths and weaknesses, with no clear winner that is best for everyone. This diversity is good, but can be confusing to users who don't know which app(s) will be best for them. I hope this review will help.

All of the seven serious star-charting apps have these basic features:

At the next level of detail, however, the seven apps are quite different. Here's a brief summary of their main distinguishing features (in alphabetical order):

In addition to these seven apps, there is a free version of Distant Suns (that lacks all the text information and the ability to set the time), plus a less capable version of Starmap (with smaller databases and fewer features, but still fairly powerful and complex).

In choosing one or more of these apps to purchase, you should ask yourself questions like these:

With these questions in mind, you can now follow the links above to learn more about each individual app, or follow the links below to see visual comparisons of some of the main features of the seven apps. Then scroll down to the Comparison Table to see a detailed side-by-side comparison.

Visual Comparisons:

Comparison Table

Distant Suns GoSkyWatch iAstronomica iStellar Starmap Pro Star Walk Uranus
Creator Mike Smithwick GoSoftWorks Artistic Techworks AstroArts Frederic Descamps Vito Technology Deep Prose
Version 1.2.1 3.1 1.0 1.1.2 1.1 2.0 1.0.2
Price (USD) $5.99 $9.99 $8.99 $8.99 $18.99 $4.99 $9.99
Size 7.1 MB 10.7 MB 0.5 MB 2.8 MB 57.2 MB 22.3 MB 7.7 MB
Startup Time 10 sec 6 sec 4 sec 7 sec 11 sec 8 sec 15 sec
Drag/Zoom Speed Good Excellent Good Good Good Excellent Fair
Max Field of View 100° 180° 180° 115° 90° 90° 80°
Min Field of View < 0.01° 18° 0.08° 18°
Adaptive Zoom DSO names None Constellation names Fainter stars and names Faint stars and DSO's Constellations, star names Star and DSO names
Screen Orientation Either Portrait Portrait Either Either Landscape Portrait
Sky Color Black or Sunlit Day/Night Day/Night Day/Night Adjustable Day/Night Black
Horizon Opaque or Transparent Transparent Opaque Opaque Opaque or Transparent Transparent Transparent
Time Animation Yes Yes Yes Yes No Yes No
Search Objects Planet, Const., DSO Planet, Const., Star, DSO Planet, Const., Star Planet, Const., Star, DSO Planet, Const., Star, DSO Planet, Const., Star, DSO None
Search Method Select from list Type name or select from list Type name Select from list Type name or select from list Type name or select from list None
ID Method Center object Center object Tap Turn names on Tap Tap Tap + Slider
Planets Drawn As Colored Dots Stars & Circles Colored Dots Stars Images Images Symbols
Planet Info Very extensive Extensive Dist., RA-Dec Name only Very extensive Physical data RA-Dec
Meteor Showers No No Yes Yes Yes Yes No
Constellation Lines On/Off On/Off Always On On/Off On/Off On/Off Dimmable
Constellation Borders On/Off On/Off No No No No Dimmable
Constellation Pictures No On/Off No On/Off No On/Off No
Magnitude Limit 4 or 6 0 to 6.5 4 1 to 7 -2 to 16 8 0 to 10
Total Stars 5000 8000 500 16,000 2,500,000 40,000 120,000
Identified Stars All All Most 254 8900 8000 All
Star Colors No Yes No Yes Yes Yes No
Star Info Very extensive Extensive RA-Dec Name only Extensive Mag., RA-Dec Mag., RA-Dec
Deep Sky Objects 110 219 None 110 13,200 110 500
DSO's Shown As Colored dots Symbols None Symbols Symbols Images Symbols
DSO Info Very extensive Extensive None Name only Extensive Mag., Dist., RA-Dec Magnitude, RA-Dec
Photos Yes Yes No No Yes Yes No
Alt-Az Grid On/Off On/Off Quadrant lines On/Off On/Off No No
Equatorial Grid Equator On/Off On/Off No No On/Off No Always On
Ecliptic On/Off On/Off Always On No On/Off Always On Always On
Night Vision Mode Menus only Chart and Menus None None Chart and Menus Chart and Menus Chart and Menus
Accelerometer Pointing No Yes No No Yes No Unusable
Help Screen(s) Yes No Yes No Yes No No
Other Notable Features Event alerts, Constellation descriptions, photos Photos of planets and deep sky objects Lists current events Comets, twinkling stars Tools for telescope users & photographers Photos of planets and deep sky objects Dimmer control for labels and lines
Bugs and Problems Faint stars hard to see; toolbars too crowded Text clutters chart Star positions are inaccurate; time animation is jerky Too little information Complex; some time calculation bugs Some bad DSO data; too little control Slow graphics, limited zoom range, no search
Ease of Use
Info Rating
Fun Factor
Detailed Review Distant Suns GoSkyWatch iAstronomica iStellar Starmap Pro Star Walk Uranus

Size is listed here to give some idea of how much information the app actually contains--with photographic images taking up the most space. Even the largest of these apps takes up less space than half an hour of music, so don't worry about running out of storage.

Startup Time is especially important if you often use the app as a quick reference, for instance, to look up when the moon will rise. These times were measured on a first-generation iPhone.

Drag/Zoom Speed is my subjective impression of the app's graphics performance. Only Uranus falls short in this category, thanks to the improved graphics in more recent versions of Starmap. Three of the apps (GoSkyWatch, iStellar, and Star Walk) even give "inertia" to the sky chart, so it coasts along for a moment after a fast swipe across the screen.

Field of View indicates the zoom range--the wider the better. Only two of the apps will show you the whole sky at once (180°), while most of the apps don't zoom in quite as far as I would like. (Typical binoculars show a field of about 7°, while a small amateur telescope might show a 1° field.)

Adaptive Zoom indicates whether certain objects or text appear and disappear as you zoom in and out. Some apps use this feature in an essential way, while others give you full control through manual settings.

Screen Orientation can be changed in some of the apps by simply rotating the device, thanks to the built-in accelerometer.

Sky Color indicates how the background sky is drawn. Four of the apps will change the sky color from black (or very dark blue) at night to blue in the day. Of these four, iStellar hides the stars during the day (unless you turn the daylight completely off), while the others still show the stars (as I prefer). Distant Suns will optionally lighten the sky around the sun, but in an artificial way that doesn't affect the whole sky. Starmap Pro lets you adjust the sky color from black to light blue regardless of the time of day.

Horizon refers to the horizon line and the ground below. My personal preference is for a transparent horizon so I can look at objects below it. The horizon in Uranus is so transparent that it's difficult to make out at all.

Time Animation is almost essential if you want to visualize and understand how the sky changes over time (without actually waiting for the hours and days to pass). All the apps that have this feature let you run time backwards as well as forwards.

Search capability lets you find an object in the sky by either typing its name or selecting the name from a list. Search targets can include planets, constellations, named stars, and/or deep sky objects (star clusters, nebulae, and galaxies). You can also search for comets in iStellar, and for meteor shower radiants in iStellar, Starmap Pro, and Star Walk.

ID Method indicates how you use the app to identify an object that it shows. Alternatives include tapping on the object, moving the chart to center the object, or simply looking at a name displayed next to the object.

Planets are drawn in different ways by the various apps. Two of them draw planets as stars, to show how they actually appear to the naked eye. Three of the apps use colored dots or symbols to represent the planets, presuming that you already know what they look like. The other two apps draw photographic images of the planets, showing more detail than you would see even in a telescope. The apps also vary greatly in how much information they give you about the planets.

Meteor Showers occur on certain dates each year. Four of the apps provide lists of the principal meteor showers and their dates. Star Walk even draws simulated meteors zipping across the sky.

Constellations can be shown as lines connecting the stars (to make stick-figures), lines marking the official constellation boundaries, and/or pictures showing an artist's conception of what the constellation represents.

Magnitude Limit indicates the faintest stars that are shown, with higher numbers indicating fainter stars. In urban areas, or under a full moon, you'll rarely make out stars fainter than magnitude 4. Under ideal conditions you can see magnitude-6 stars with the naked eye, and magnitude-10 stars with good binoculars. For some of the apps the magnitude limit is not published, so I've estimated it; for most apps the magnitude limit is a little fuzzy. The magnitude limit determines (roughly) the number of stars that the app will show. The number of stars that the apps will identify (by name or number) is sometimes much lower. For identified stars, most of the apps will also tell you the coordinates, magnitude, and/or distance. Distant Suns will even tell you the type of star and how fast it is moving.

Star Colors are pretty to look at and also indicate the approximate surface temperature of a star, with orange-red being coolest and blue-white being hottest.

Deep Sky Objects include star clusters, nebulae (gas clouds), and galaxies. Only a few of these objects can be seen with the naked eye, while dozens more can be seen (faintly) in binoculars. Three of the apps use the famous list of 110 DSO's compiled in the late 1700's by Charles Messier. While this list includes most of the more prominent DSO's, it omits several dozen that are equally prominent including all of those that cannot be seen from northern latitudes. Three of the apps go beyond Messier's list to include other DSO's. Most of the apps show DSO's using various abstract symbols, although Star Walk shows a magnified photograph of each individual object. As usual, Distant Suns provides far more text information about the objects than the other apps.

Photos of planets and deep-sky objects are included in four of the apps. (Starmap Pro provides photos only of the Messier DSO's, not the other 13,000 in its database.)

Sky Coordinates can be shown in two different systems, each somewhat analogous to latitude and longitude angles on earth's surface. One system is Altitude-Azimuth, in which the altitude is measured up from your current horizon and the azimuth changes as your gaze moves horizontally around the circle. Because your horizon moves as earth rotates, the alt-az coordinates of astronomical objects are constantly changing. The other system is Equatorial coordinates, called Right Ascension (analogous to longitude) and Declination (analogous to latitude), with the celestial equator being a great circle around the sky directly above earth's equator; in this system the coordinates of stars are essentially fixed, while the coordinates of the sun, moon, and planets change gradually over the days and months.

The Ecliptic is a great circle around the sky following the sun's apparent path among the stars. It is tipped 23.5° from the celestial equator, due to the tilt of earth's spin axis with respect to its orbital plane around the sun. The moon and planets are always seen within a few degrees of the ecliptic, because their orbital planes are tipped only slightly with respect to earth's. The ecliptic passes through the twelve constellations of the Zodiac.

Night Vision Mode is a feature that dims and reddens the display, to avoid spoiling your eyes' adaptation to the dark.

Accelerometer Pointing is a feature in GoSkyWatch and Starmap Pro that allows you to hold your device above you and tip it to indicate the direction you're facing; the chart then updates, based on the tip angle, to show the sky in that direction. Some users will probably love this feature, while others will rarely use it. Uranus attempts to implement a similar feature, but I can't get it to work.

Help Screens within the app are provided only by Distant Suns, iAstronomica, and Starmap Pro. For the other apps you have to either look at the creator's web site for instructions or figure things out on your own.

Ease of Use is my subjective rating based on the level of expertise (both in astronomy and in using the device) that's required, as well as how conveniently users can access the app's features. Pay careful attention to this rating if you're a novice who learns slowly; ignore it if you're a geek who already knows some astronomy.

Info Rating is my subjective rating of how much astronomical information the app provides. One star in this category may be enough for those who just want to learn the planets and constellations; more stars means more objects and/or more information about them.

Fun Factor is my subjective rating of how much fun you'll have playing with the app, based on graphics quality, graphics performance, range of features, and user interface design. Some people learn more when they're being entertained, while others may find it distracting.


I find it remarkable that iPhone/iPod Touch developers have created so many powerful star-charting apps in just one year. None of these apps were developed by big software companies, and several of them seem to have been written by a single programmer. We owe all the developers great thanks for their impressive efforts, which have given us a rich diversity of software to choose from.

If history is any guide, however, this diversity won't last. Some of these apps will improve over time, while others will stagnate and eventually become incompatible with newer hardware or system software. Let's hope that the surviving apps continue to offer most of the features that are available now.

For the short term, each of these apps can be improved in minor ways that will benefit all users. Improvements to an app will also help it survive in the face of such strong competition.

It will be interesting to see whether any of these apps, or others that come along, will try to implement even more ambitious features such as a 360° sky view in Mercator projection, or smoothly changing latitude and longitude while viewing the chart, or viewing celestial objects from locations other than earth. Presumably there are limits to what can be done (or at least, done well) on the small screen of the iPhone/iPod Touch. But creative software designers have already shown that those limits are far beyond what many of us expected.

I anxiously look forward to seeing the next round of bug fixes, enhancements, and new releases of astronomy software for the iPhone and iPod Touch. And I encourage you to support the developers' continued efforts by purchasing as many astronomy apps as you will use.

Other Astronomy Apps for iPhone and iPod Touch

In passing I should mention that there are several other astronomy apps that don't really belong in the same category as the eight reviewed here. At least five others provide some basic star-charting capability:

And finally, there are several astronomy apps that are useful even though they don't draw star charts:

Please help!

Although I hope this review will be useful, it is bound to contain errors and important omissions. Please
contact me if there is important factual information that I have overlooked or gotten wrong. I will try to update this review as appropriate, to reflect the latest information and revisions.

Revision history

2 July 2009: Updated for Starmap Pro and version 3.1 of GoSkyWatch.

16 February 2009: Updated for version 1.1.1 of iStellar, which adds more stars, comets, and search capability. Minor updates for the bug fixes in version 1.2.1 of Distant Suns.

4 February 2009: Updated for version 1.21 (1.2.1?) of Star Walk, which adds meteor showers and other minor features.

23 January 2009: Updated for version 1.2 of Distant Suns, which adds time setting controls and alerts.

12 January 2009: First version of this review.

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