Rewrite each original sentence in order to make it easier to understand by non-native speakers of English. You can do so by replacing complex words with simpler synonyms (i.e. paraphrasing), deleting unimportant information (i.e. compression), and/or splitting a long complex sentence into several simpler ones. The final simplified sentences need to be grammatical, fluent, and retain the main ideas of their original counterparts without altering their meanings.

The winter solstice occurs at the instant when the Sun's position in the sky is at its greatest angular distance on the other side of the equatorial plane from the observer's hemisphere.
When the sun is furtherst from the oberver the winter solstic is occuring.