The Sky Guide: View Stars Night or Day ($2.99) astronomy iPad app is notable for the sheer beauty of its depiction of the starry sky. It does a good job of capturing gradations in the sky's hue and brightness as you move your eyes from the horizon up to the zenith, and in depicting the great star clouds, star clusters, and nebulae in the Milky Way, as well as other nearby galaxies. But Sky Guide doesn't identify as many faint stars as SkySafari 3 or Luminos - Astronomy Companion, both Editors' Choice astronomy apps. It also shows far fewer asteroids and comets, making it best for casual stargazers.
Design and Features
As a so-called planetarium program, Sky Guide displays a realistic view of the starry sky in the direction you are pointing the device. In fact, the view is dazzling, probably the most visually stunning of any such app we've tested. As the Sky Guide's full name suggests, it renders the starry sky even in the daytime. If you hold your iPad in the direction of the Sun, you will see our star as a very bright spot against the background constellations through which it is passing. This ability to render the sky at which it is pointed, while very striking, is not unusual; in fact, it is typical of planetarium apps.
What is less typical is that the app uses the full screen to display its virtual heavens. There are controls, but to reach them, you have to tap the screen. When you do so, you see an icon composed of three stacked horizontal bars, for accessing the Settings menu, and a magnifying-glass icon that links to the Search menu. If you don't press either of these icons, they disappear from the screen in about four seconds. Sometimes, especially when your iPad is resting screen-up on a flat surface, you may also see a compass icon to the right of the settings icon. Touching the compass lets your iPad's internal compass (and, in some cases, GPS) correctly orient the view. You can also enable the compass simply by tilting your iPad upward.
Navigation is easy enough, with a little practice. To pan right, you move your finger across the screen to the left; to pan left, you swipe to the right. To pan upward, you move your finger downward, and to pan downward, you move your finger upward. To go to a wider-field view, you pinch the screen, and to zoom, you stretch. Sky Guide doesn't have as large a zoom as some other apps, however. At maximum zoom, the Pleiades star cluster fills about a third of the field of view, while in SkySafari and Luminos, it fills the entire field. To go to a specific constellation, star, or planet, you can use the Search function, which I'll discuss later.
I tested Sky Guide on an iPad Air 2, to take advantage of the tablet's relatively large screen. The app is also compatible with the iPhone and iPod touch, and there's even a version is for the Apple Watch.
The Settings menu contains six items: Preferences, Time & Date, Location, Notifications, Help, and Share. The Preferences menu lets you turn certain features on and off, including Constellations, Mythology, Ecliptic, Horizon, Labels, Satellites, Music, Sounds, and Night Vision. By default, all these functions are turned on except Ecliptic, a line that depicts the path of the Sun through the constellations over the course of a year, and Night Vision, which in effect puts a red filter over the screen while reducing its brightness, to help preserve your eyes' dark adaptation should you be using the app outside at night.
The Constellations feature adds the lines between stars that are commonly used to represent constellations, while Mythology depicts the (largely mythological) figures and objects that the constellations represent. These figures are beautifully presented. The Horizon setting shows the horizon and darkens the ground, although you can still see the stars and constellations that lie below your horizon. The Satellites item shows and labels bright satellites, which you can follow as they move across the app's virtual sky. Music is ambient space music, while Sounds are various tones that you hear when you tap objects. Most stars, except the very faint ones, emit a tone when you tap on them. The redder and cooler the star (in the app, the stars are revealed in their true color), the lower the pitch, while hot, blue stars give off a high-pitched tone. You can even create music of sorts by tapping on a sequence of stars.
With the Time & Date feature, you can use the current time (the default), select a date and time from a scrollable calendar, or move backwards or forwards in time at a speed of your choosing. The Location feature defaults to your current location, but lets you choose Manual Location, which brings up a list of countries, and then cities within the country that you choose. In the Notifications section, Astronomical Events (notifications of meteor showers, eclipses, and so on) is turned on by default, while you have to enable Satellite Passes, which sends you notifications of International Space Station passes and bright Iridium satellite flares.
When you click on the Share function, the last item in the Settings menu, a large button appears at the bottom of the screen. Tapping it takes a screenshot, and opens up a dialog box to let you share the image via text message, email, Twitter, Facebook, or Flickr, as well as copy the image, save it to your Photo Album, or print it out.
The Search button opens a menu with a search field on top, below which are seven categories: Constellations, Stars, Solar System, Comets, Deep Sky, Satellites, and Favorites. Tapping a category reveals a drop-down list of its contents. In any of the search categories, you can navigate to objects even when they are below the horizon; their names will appear in gray. From the Constellations menu, you can go to any of the 88 constellations; the time until they set (or rise) is listed. Stars by default are sorted by brightness, although you can also sort them alphabetically.
Noted author James Kaler's descriptions of some of the brighter stars are both informative and eloquent. But for many of the other star descriptions whose author is not named, the information is generic, and in at least one case lacking in important details. The entry on Albireo, the star that marks the beak of Cygnus, the Swan, notes that the star is a yellow-orange (K-type) giant star that shines with the energy of about 1,800 Suns, and after shedding its outer layers it will end its life as a white dwarf star, surrounded by a so-called planetary nebula. The account fails to mention that even the smallest telescope will reveal that Albireo is not just one star but a binary (double) star, with the yellow star accompanied by a blue companion. In fact, Albireo is one of the showpieces of the summer sky, and is mentioned as such in countless astronomy columns and books. Albireo is actually a triple-star system, as the yellow star itself has a very close companion, but it's beyond the reach of amateur telescopes.
Speaking of amateur telescopes, it's worth noting that Luminos actually gives you the ability to guide a computer-controlled telescope; Neither Sky Safari nor Sky Guide can do so.
The Comets section currently lists 17 comets. Several are currently visible, while most are historical objects, like Halley, ISON, Hale-Bopp, and Hyakutake. For these objects the tutorial notes that you can call up a comet, touch its information button, and find its date of last perihelion (when it was closest to the Sun). You can then set the app's time to that date and see the comet when it was at its brightest. The number of comets shown in Sky Guide is paltry compared with that of SkySafari 3, which shows more than 800. Its selection of asteroids, found in the Solar System section, is equally meager. SkySafari shows more than 100, while Luminos shows more than 37,000 comets and asteroids. Granted, many of the objects shown by SkySafari and Luminos aren't currently observable, and most casual stargazers will be fine with the few handfuls of comets and asteroids that Sky Guide provides.
Sky Guide does provide an ample selection of more than 200 satellites. In addition to sorting them alphabetically, you can sort them by pass, a chronological list showing when each satellite will pass overhead and be brighter than magnitude 4 (bright enough to be seen with the naked eye in suburban skies).
A Stellar App for Casual Stargazers
The Sky Guide: View Stars Night or Day iPad app is very appealing, particularly for more casual stargazers. It beautifully renders the night sky in one of the most dazzling views we have seen. It's at least as appealing in this regard as Sky Safari 3, and more so than Luminos, our most recent Editors' Choice planetarium app. Sky Guide doesn't show the sheer number of celestial objects as these other apps, making it of limited use to advanced amateur astronomers. Furthermore, it doesn't have the detailed information on many stars and other objects found in these other apps. But this visually stunning app is a good choice for anyone interested in finding their way around the night sky, or in the aesthetics of the cosmos.