Plan a Milky Way shot on iPhone

Three conditions, and they have to hold at the same moment

Almost every failed Milky Way night is one of three things, and the three are independent. Knowing which one stopped you is the difference between waiting a week, waiting a month, and driving south.

1. The galactic centre has to be high enough

   Sunlit treats 10 degrees of altitude as the floor. Below that the core sits in the thickest and dirtiest part of the atmosphere and there is nothing worth exposing for. It grades what you get above the floor: around 15 degrees is fair, 20 is good, 30 and up is excellent.

2. The sky has to be astronomically dark

   That means the sun more than 18 degrees below the horizon. This is the condition that quietly kills the northern summer: above roughly 48 degrees north there are weeks when the sun never gets that far down and the night never properly ends, no matter how clear it is.

3. The moon has to be out of the way

   A moon that is up and more than about 30 percent illuminated washes out the frame. Sunlit treats that as the tolerance, and a bright moon during the dark window drops the grade by a step.

The app names the one that is blocking you

A planner that only says "not visible tonight" has told you nothing you can act on. When there is no window, Sunlit says which of four conditions closed it:

• The centre never rose high enough. The answer to this one is geography. Drive south.
• Twilight. The night never got dark. Wait a month.
• Moonlight. Every dark hour the core was high had a bright moon in it. Wait a week.
• Season. The core was high and the night was dark, but never at the same time. That is the wrong half of the year for your latitude.

When there is a window, you get its start and end, the best moment inside it, the altitude of the core at that moment, and a grade.

Then find it in the sky

Hold the phone up. Once the selected instant is dark enough, the galactic plane is drawn over the live camera with the galactic centre marked on it, alongside the sun and moon paths. It is not drawn over a blue sky on purpose, because a Milky Way painted across daylight is a promise the sky will not keep. Use it in the afternoon to work out where to put the tripod, and come back after dark knowing the composition already works.

The position of the centre comes from Sagittarius A star, precessed to the date, which places it to about 0.01 degrees. The AR guide explains the heading uncertainty chip, which matters more at night than it does in daylight, because there is nothing on the horizon to sanity check the direction against.

What Sunlit does not do

There is no cloud forecast in it, no light pollution map and no image processing. It tells you where the core will be, when the sky is dark, what the moon is doing, and what your own measured skyline is hiding. The weather is still your problem, and so is finding a dark site.

What this costs

The Milky Way, the moon and any date other than today are part of Sunlit Pro, one payment of 9.99 EUR with nothing renewing. The free version covers today's sun at the place you are standing, in all four views, which is enough to scout a foreground in daylight.