You can't find SkySafari 1.8.5 for Android if you search for it. But, you can still access it from your "My App" section of the Google Play Store. Any apps purchase will be there.
However, you must be logged into the store using the same Google Play account that was used to make the original purchase SkySafari.
If you can't find the app in Google Play on your mobile device try logging into Google Play using a web browser on your computer or installing an app called Purchased Applications.
You are not required to repurchase SkySafari 1.8.5, but you do have to find it in Google Play "MyApps" category. There is nothing else we can do about helping you reinstall the previous purchase. SkySafari 1.8.5 has not been delisted for existing customers. If you bought it you can find it.
If you login with the correct Google Play account, and still can't see your previous purchase of SkySafari then you will need to contact Google Play store support.
Experimental (untested solution):
You might try clicking the link below for the app you previously purchased - provided you are logged into your Google Play store account that was used to make the original purchase the link below may open up the app for you to install:
SkySafari Pro 1.8.5 (was approx $39.99 USD)
SkySafari Plus 1.8.5 (was approx $14.99 USD)
SkySafari Lite 1.8.5 (was approx $2.99 USD)