Describing facts in a story written in present tense

Could anybody tell me which is more appropriate?

"A day in Titan is about 383 earth hours"

or

"A day in Titan was about 383 earth hours"

when these words are used to describe the setting of the novel written in past tense?

I feel like "is" should be used because what it describes is an everlasting fact but using it makes it feel more like a response to a question than the description of the setting. Please excuse me as my first language is not English , I just want to use writing as a tool to improve my language.

When we are talking about past events, for things that were true at the time when the story occurred, and that are still true at the time that we are writing the story, we say "is". Like if I was writing a story today that is set in 1750, I might write, "Virginia is in North America."

If something was true at the time the story occurred but is no longer true, we say "was". Like in that same 1750 story I might write, "Virginia was a British colony." If it's necessary to make clear that this is no longer the case, we add additional words. "At that time, Virginia was a British colony."

In the case of a fiction story, what matters is when the story is set and when the narrator is supposed to be telling the story. If the date of narration is not obvious or important, I'd say that things that would be expected to be true for a very long time, like the length of a planet's day, should be in present tense. Things that might change regularly, like the weather or the name of the governor, should be in past tense.

To expand on Jay's answer, it depends on the narrator's POV.

If the narrator is omniscient and relating facts that are both current and historical, like a textbook or historian, this voice "knows" what is in the past and what is current. This voice will reflect the time of the reader as if the narrator is speaking directly to him/her. "A long time ago, in a galaxy far, far away…" – the narrator is omnisciently aware the reader is not in the same era or location as the story. This "narrator" can have it's own character personality and speaking style, and this voice might interrupt the narrative to take the reader on a side topic, like the voice of the book in Hitchhiker's Guide to the Galaxy a spoof of a travel guidebook full of useless factoids about the location.

Limited 3rd-person POV reflects one character's experience (at a time), and is limited to their perception of time and place. This "narrator" will not break the 4th wall to signal things to the reader that the character himself doesn't know, but also this narrator will not stop to explain things to the reader which are obvious to himself. He would never casually say factoids to himself, like "A day on Titan is about 383 earth hours." Instead he would either discover it within the novel and the reader become informed through his experience, or it might come up in conversation with another person in the book who needs to learn it.

1st-person POV might be a semi-omnicient narrator who speaks directly to the reader long after the events in the story: "As we discovered later, a day on Titan is about 383 earth hours, but we had no way to know it at the time." Or 1st-person POV might be a form of epistolary novel which limits his knowledge to the moment when he writes it down: "Dear Journal, I signed up for the night-shift before realizing a day on Titan is about 383 earth hours. I have an appointment to speak to my supervisor tomorrow, but I don't know if that means in 7 hours or 140!"