Why Spring is Galaxy Season

I’ve been hard at work on the second edition of The Deep-sky Imaging Primer, which is the main reason I haven’t posted anything new in such a long time. (The other reason is that we have had a very long stretch of astronomically unfriendly weather here in NJ.)

In the second edition, I am adding some sky maps focused on imaging-worthy objects. After many hours of programming and data munging, I have some useful charts. One of these is a simple plot that clearly explains why we call spring “Galaxy Season.”

Imaging-worthy galaxy locations by optimal viewing date

So what are we looking at here? On the X-axis is declination. Up at the top you can see a distorted little dipper, because this is an Mercator-like projection (orthographic, if you really want to know). On the Y-axis, instead of hours of right ascension, I’ve marked the map by the date of ideal viewing–that is, the night that that particular line of RA is directly overhead at solar midnight. In other words, these are the objects that will spend their maximum time in the night sky on the date at the bottom.

And what is plotted in red? Those are galaxies that I’ve deemed to be at least remotely “imaging-worthy.” They don’t have to be pretty, they just have to be at least 5 arcminutes across their longest axis. Pretty simple, huh?

Because our own galaxy blocks our view of other galaxies, the time of year we can see the most galaxies in the night sky is when we see the least of our own. This is in the spring and fall, and particularly in the northern hemisphere spring. In fact, you could probably estimate that the absolute peak for galaxy viewing and imaging is sometime in late March for those of us in the north, as this is when the Virgo and Coma groups are in prime position. So get your long focal length scope ready and cross your fingers for good weather, as Galaxy Season is upon us!