One drawback to the many iPhone astronomy apps is the lack of large databases. To an advanced amateur such as myself, the lack of detailed databases is a distinct negative in these programs. I venture way beyond the Messier catalog, or even the NGC on a regular basis. I understand the price to be paid in speed and memory required to support larger databases, but I would like to have the option.
The programs I have seen also lack the ability to display comets or asteroids. This last issue has become a problem lately, as we have several nice comets available in the sky for observing. A quick way to look up the current position for the object is essential for comets and asteroids if you wish to observe them. Last night’s coordinates will have you looking at an empty starfield tonight.
There is one decent answer for comets… I found a very nice app for comet ephemerides published by Keith Yohn. Called simply Comet, this free program downloads the latest ephemeris data from the Minor Planet Center and displays the current and future coordinates for each comet. Just what I needed!
There are no charts, the data is simply displayed in table form. Every observable comet is displayed and can be search for using a simple set of filters.
When attempting to observe comet C/2009 P1 Gerradd a few times recently. I have found the coordinates to be quite accurate. The comet was in the sky, right where the app showed it would be. Nothing like a real world test.