Are observatories complete this sentence. This question is about inversion, every starting sentence with a negative expression such as rarely, seldom, hardly, never, no, etc. subject and verb must be inverted.
In addition, the above sentence starts with a negative frequency, and the sentence is omitting the fundamental subject as a possible segment of the main verb.
The negative adverb at the start reveals an inversion between the subject and verb. In this sentence, always pay close attention to the subject, verb, and accord.