Now that smartphones and tablets have become ubiquitous many people ask us why we don’t publish a planisphere app instead of continuing to sell physical planispheres. The answer is that, in this case, there are several advantages to the physical format. We have used most, if not all, of the apps available on the iTunes store and agree that they are quite amazing and a lot of fun to play with. There are some that are better and some worse, but they all have a few ingrained limitations that the format just can’t quite overcome.
- One of the most important limitations is that the LED backlighting that makes phone displays work is very rich in short wavelength blue light. Even with the brightness turned down and using the “red mode” included with many apps there is a surprising amount of blue and near UV light being beamed straight into your eyes. While this is never really a good thing, it is of particular disadvantage to your night vision. One recent night here in suburban Denver (fairly high degree of light pollution) I was observing the Pleaides and took out my iPhone to test a new app. The app had the location fairly close, although when I looked up at the sky again (I was using the “red mode”) I found that I could not even see the cluster again for several minutes.
- Unfortunately, many of the apps we have seen suffer from quite a bit of distortion. When this is the case it is very difficult to see in the sky what you see in your phone display. This can be consummately frustrating to beginners.
- A physical planisphere shows you all the context at once. Rather than just the narrow view from your phone screen you can see the adjacent stars and constellations without having to swing your arm back and forth repeatedly.
- A physical planisphere can “time-travel”. You can quickly and easily move time up and back to see where objects will be in the sky at different times. You can get a much better sense of the apparent movement of objects across the heavens simply by rotating the planisphere setting back and forth.
- As we just mentioned, this “time-travel” ability can be used plan ahead which is particularly helpful in setting up for astrophotography shots or in planning a list of Messier objects for an evening’s observing, or simply to find out if a particular object you are interested in will be visible on a particular night and time.
- Finally, you can see in one glance if our planisphere is configured for the correct date and time. With those settings in the background of an app it can be hard to tell if you have the incorrect timezone or a poor GPS signal. (A physical planisphere also will never quit because of a dead battery!)