I was walking with friends one evening back in May as twilight deepened. I knew Venus would soon be visible, though I wasn't quite sure where to look for it. I took out my iPhone, loaded the Star Walk app, and held the phone up to the sky. As I moved my phone, Venus soon appeared on the sky map the app displayed, and sure enough, I quickly located the planet in the sky.
Star Walk 6.0 (for iPhone) ($2.99 through iTunes store) is a kind of app known as a planetarium program?though it doesn't need a dome or projector, like a planetarium it provides a representation of the night sky showing stars, planets, nebulae, and other celestial objects visible at a given time and date from a location. In a sense the sky itself is Star Walk's dome, as you can hold your iPhone up to the night sky and it will reveal the celestial wonders that lie in the direction you're looking.
Star Walk for the iPhone has one advantage over Star Walk 6.0 (for iPad) ($4.99): phones are more portable than tablets, so you're more likely to have your iPhone with you when you find yourself out under the stars. Otherwise, they're essentially the same app.
When you open Star Walk, you first see a screen called Sky Live, which gives you information on the Sun, Moon, and the planets Venus, Mars, Jupiter, and Saturn for the current date (by default, though you can change the date). This data includes rising and setting times and the maximum elevation in degrees for each object. In addition, it gives the Moon's phase and how close to Full (by percentage) it is. (For the latest on the Sun, the dazzling free 3D Sun app for iPad and iPhone is worth getting.)
The Virtual Heavens
When you close Sky Live, a map of the night sky is enabled, revealing constellations (which are labeled), the planets, and our own Milky Way galaxy, shown as a bright band, broken by dark dust clouds, that spans the heavens.? Also marked are the ecliptic (the apparent path that the Sun traces against the constellations over the course of the year as the Earth circles it) and?at the horizon?markers for each of the four cardinal directions. Stars below the horizon are visible, though obscured.
You navigate Star Walk's virtual sky by pinching, stretching, and swiping. You can pan horizontally or vertically by swiping. By stretching, you can zoom in on smaller sections of sky, revealing more stars; many of the brighter ones are labeled with their names. Tapping on any star will reveal its name, even if it's a prosaic designation like HIP 103976, and an information icon. Touching the icon calls up information such as the star's visual magnitude (brightness), altitude and azimuth as well as celestial coordinates, distance, and spectral type. Pinching the screen brings you back to the wider sky view.
In daytime, Star Walk shows the Sun, with the constellations and planets still visible. (In reality, only the Moon and?on occasion?Venus would be easily visible to the unaided eye in daytime.) Day or night, the constellation at the center of your screen is depicted with lines connecting the most important stars, and with an illustration of the mythological or other figure it represents.
?With the Star Spotter function that utilizes the iPhone's digital compass, Star Walk is able to show the constellations visible in the direction you are holding your tablet (more or less). This is true even if your phone is not getting any data reception. What's more, when you tilt your iPhone upwards, by touching the camera icon in the screen's upper right corner the star map will appear translucent and superimposed on the actual night sky as seen with your iPhone's camera. How many stars you actually see largely depends on whether you live under dark rural skies or (like me) in an urban environment where relatively few stars are visible. Even without the translucent view, though, being able to see the constellations on your iPhone's screen in the direction you're pointing it is a great feature (though one that's standard with planetarium apps).
Where Size Should Matter
One flaw is that there's no way to tell a planet's brightness in Star Walk. Stellar brightness is (more or less) depicted by the size of the star's image, and a numerical representation of its brightness (visual magnitude) is also accessible by clicking on the image. Neither is true with planets, though; all the planets are shown at roughly the same size (with Jupiter and Saturn slightly larger than the others, and Pluto?still labeled as a planet, as it is in many astronomy programs?a little smaller), with not even a magnitude figure given. In reality, the planets vary tremendously in apparent brightness, with Venus roughly 25 million times brighter than Pluto, and the brightness of each planet also varies over time depending on where it is in relation to Earth and Sun. Many apps have similar problems with planet brightness; one that does better is GoSkyWatch Planetarium ($3.99, for iPhone and iPad).
?One evening I went to a park to try to see Mercury?as the innermost planet, it's visible infrequently, usually in bright twilight. It would be fairly near the bright stars Castor and Pollux in Gemini, all very low in the west.? I was briefly able to glimpse a single object shining in a gap between trees; I suspect it was Mercury, but it could have been Castor or Pollux, and the app gave no hint as to how bright Mercury should have been that night relative to these and other stars.
The depiction of star brightness is also problematic in Star Walk. Stars may appear the same size even when there's a tenfold or more difference in brightness between them. Sirius, the brightest star in the night sky, is one of five bright stars in Canis Major, but it looks no brighter than the other four, even though it outshines each of them by at least 15 times. (Its spikes are longer, but the others appear to glow more, so Sirius actually looks slightly fainter than the others.) Still, Star Walk does a credible job of depicting the constellations despite this issue, and it's a problem the app shares with most consumer-level planetarium programs to a greater or lesser degree. StarMap ($4.99, for iPhone and iPad) is one that does better with brightness.
Ghostly Galaxies
Close-up views also reveal small images of a number of deep-sky objects?galaxies, star clusters, and nebulae. Many well-known objects (from the Messier and NGC catalogues) are shown, as well as some fainter objects that would be difficult to find in a small telescope.? At least one conspicuous object is missing altogether: the Double Cluster in Perseus, a close pair of star clusters that is one of the showpieces of the fall and winter sky.
One caveat is that the image's size and brightness is unrelated to the object's prominence in the night sky. For instance, the Andromeda Galaxy is in the same (mid-zoom) field of view as the spiral galaxy NGC 278, and they look equally conspicuous in Star Walk. In reality, Andromeda appears much larger in our sky, and is about 250 times as bright as NGC 278. The app does give numerical figures for stars' and galaxies' apparent brightness (visual magnitude), though casual skywatchers would not be expected to be familiar with the magnitude scale.
Speaking of the Andromeda Galaxy, mid to high zoom views show it along with two companion galaxies, but there's also ghostlike cloud nearby. Tapping on it did not reveal its identity?it was only when I pinched the image that I realized that it was a larger version of the same Andromeda Galaxy so it can be made out in wide-field views. The Pleiades star cluster is treated in the same way, as is the Small Magellanic Cloud, one of the Milky Way's two main companion galaxies. The other, the Large Magellanic Cloud is shown but not identified.
Tweeting the Sky
When you touch the "Send to" icon at the screen's upper left, you can send a screenshot to email or post it to Twitter or Facebook, save it to your camera roll, or print it out. You can also rate the app, or gift it. When I tried sending a screenshot to email, it opened a message in my default email client with the screenshot pasted in place.
Once I had authorized Star Walk to access my Twitter account and enabled tweet location to be shown, I was able to tweet screenshots. When you click the tweet icon, a window with canned text, and links to the screenshot and the app's iTunes page appears. You can edit the text to your choosing. (Before I enabled tweet location, it would automatically tweet the canned text without giving me a chance to edit it.) With Facebook, you get the same link to the screenshot's Web page, and you can enter your own text.
There is other Twitter-related function in Star Walk. Clicking on the Community tab at the screen's lower right corner takes you to a virtual globe. When you zoom in, the world is dotted with little Twitter icons. Touching one will reveal several tweets from people near that location, each with the hashtag #starwalk. You can respond to these tweets or compose your own. Whenever I tried to send a tweet, though, I got a dead-bird icon with the message "Forbidden."
Cosmic Sleuthing, Galleries, and More
In the app's lower right corner is a search button, which lets you look for 5 types of objects?Constellations; Solar System, Deep Space; Stars; and Satellites?listed in drop-down menus. Objects below the horizon are grayed out, though you can still go to them. The Solar System search includes meteor shower radiants as well as Sun, Moon, and planets; for currently active meteor showers, it will show streaks of light emerging from the radiant. For stars, you can select from a list containing stars' proper names or Bayer designation (Greek letter plus constellation name). A nice touch is that some stars orbited by their own planetary systems (exoplanets) are included. (To find out more about these new-found worlds, the Editors' Choice iPad app Journey to the Exoplanets ($4.99) is worth a look.) The app shows some bright satellites, which move across the screen as they orbit the Earth.
The Gallery section of the app takes you to space photos from five sources: Picture of the Day; ESA (the European Space Agency); ESO (the European Southern Observatory; Featured Objects; and MWS (Milky Way Scientists).? They're beautiful and informative, and provide links to where they appear online. (For more stunning images both of celestial objects and our own world, check out the free NASA Visualization Explorer iPad app.)
From Settings, you can change your location, switch to Night Mode (in which the screen is shown in red to preserve the dark adaptation of one's eyes) turn the constellation patterns on and off;? reveal or hide satellites or the spectrum bar; turn sounds or music (ambient space music) on or off; and adjust visual magnitude with a slider. That lets you show fainter stars (such as you would see from a rural location) or only the brighter ones to match the view of urban observers.
I can recommend Star Walk 6.0 (for iPhone) as a planetarium program, particularly for budding skywatchers. Avid amateur astronomers may want to opt for a more full-featured app such as SkySafari 3 Pro ($39.99, for iPhone and iPad). Star Walk shows visually appealing maps of the heavens (in the direction you're pointing your iPhone when Star Spotter is enabled), and has social media integration, image galleries, and more. It's not without flaws: it could do a better job of showing the relative brightness of objects, planets most of all, and Twitter functionality had a few hiccups. But it mostly gets the night sky right. When installed on your iPhone, it's like having the universe in your pocket, and it's well worthy of an Editors' Choice for educational iPhone apps.
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