Astronomical sighting (4)
I believe the answer is:
nova
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">astronomical sighting</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nova that I've seen before include "sun" , "Suddenly bright star" , "Star that ejects a cloud, thus becoming brighter" , "Suddenly expanding star" , "Star showing a sudden temporary increase in brightness" .)