Star Charting Apps for iPhone and iPod Touch

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:

• Planisphere is a great new app in the bargain price range: only $0.99. I like the simple user interface and the way it shows the whole sky at once. Of course, at this price you don't get everything: it won't show the Milky Way or draw stars in color or animate the passage of time, and it gives a distorted view near the horizon even when you zoom in (as does iAstronomica).
• Pocket Universe ($2.99) is an inexpensive star-charting app with some helpful features for beginners including a constellation quiz. It has most of the features you would expect, with the exception of time animation. Maximum field of view is 100 degrees, portrait orientation only. I find the user interface a little awkward, but others may like it.
• Carina Software, a longtime developer of astronomy software for personal computers, has released SkyVoyager ($14.99) and SkyGazer ($2.99). These appear to be strong contenders in their price ranges, with a good user interface and lots of text information (and photos). Both are a little sluggish on my first-generation iPhone, and neither will show the whole sky at once.

• Distant Suns has gotten a major upgrade that adds many more stars, enhanced stellar display options, constellation artwork, sound effects, and more. Meanwhile, the developer has also released Distant Suns Lite--a free version of the program. It lacks all the newest features as well as the detailed text information and photos, and gives you no way to set the time inside the app. But the price is right, and I recommend that everyone at least try it out.
• GoSkyWatch has gotten only minor upgrades since the version reviewed here. However, the price seems to have been permanently reduced to $5.99.
• iStellar has gotten a major upgrade that adds lots of information about the stars and other objects, and zooms out to show almost the entire sky (180 by 120 degrees). Its toolbars have been completely redesigned in a way that took me quite a while to get used to.
• Star Walk has been upgraded to add pictures for all 88 constellations, plus a few settings that let you adjust the stars' brightness and turn off constellation lines and pictures if desired. There's also a quick-view screen that shows today's sun, moon, and planet information at a glance.

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:

• Identifying constellations, prominent stars, and planets;
• Showing the phase of the moon and the Milky Way;
• Moving the star chart by dragging your finger across the screen;
• Zooming the chart by spreading or pinching with two fingers;
• Using the device's clock to show the sky at the current time, or setting an arbitrary date and time;
• Using Location Services (from GPS, WiFi, or cell tower proximity) to show the sky from your current location, or manually setting a location to practically anywhere on earth.

• Distant Suns provides the most detailed text information about astronomical objects, but is limited in some other ways and can be awkward to use.
• GoSkyWatch Planetarium is both powerful and easy to use, with excellent animation and the widest zoom range. However, it doesn't draw the prettiest pictures.
• iAstronomica is the most basic of these seven apps, emphasizing easy-to-see objects while sacrificing accuracy and omitting fainter stars and deep-sky objects.
• iStellar is easy to use and has beautiful graphics and animation, but provides almost no information about the objects you're looking at.
• Starmap Pro has the most features by far, plus an enormous database of 2.5 million stars and 13,200 deep-sky objects, but is somewhat complex and expensive.
• Star Walk is visually stunning, with well-integrated photographic images and smooth animation. Its interface is especially simple, but it gives the user little control over what is and isn't shown.
• Uranus is a digital magnitude-10 star atlas with 500 deep-sky objects, intended for experienced observers. It has a rather limited zoom range, slow drag/zoom response, and no search capability.

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

• How much astronomy do I already know, and how much do I want to learn?
• Do I care more about constellations, stars, planets, the moon, or deep-sky objects (i.e., clusters, nebulae, and galaxies, most of which are visible only with binoculars or a telescope)?
• Which features are essential to me, and which are optional?
• Do I want an app that's especially informative, or especially fun to play with?
• How many of these apps am I willing to buy, at a price of up to $18.99 each?

Visual Comparisons:

	Distant Suns	GoSkyWatch	iAstronomica	iStellar	Starmap Pro	Star Walk	Uranus
Icon
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	5°	< 0.01°	18°	3°	0.08°	7°	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.

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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