Avaria is in fact a spherical body, and the lands we know in-game are in one hemisphere of it!
So there is an equator, and "tropics" that cartographers and sailors know of. The Avarian equator passes along the southern edge of Idiri -- there is actually another sort of demi-continent south of that is often simply called "south Idiri" by those who haven't spent much time there (which is most people). The equator also cuts through the northern part of Jalanjhur. So most of the lands that our Sirdabi Caliphate people are familiar with have the same seasons or else an equatorial climate.
A tropic is called a "madaar" in Sirdabi, but this can just as well be translated as "tropic," "orbit," or "band". The northern one is Madaar al-Jiniyyah, which passes through the south of the Great Hazari Desert. The southern one is the Madaar al-Darshali, and is... a long ways away! For roleplay purposes, it would be fine either to call them by those names, or use Band/Tropic/Orbit of Jiniyyah/Darshali. (Those are just the Avarian zodiac signs that the sun would be located in while directly overhead at noon on the solstices.)
The concept of meridians and parallels exists in our part of Avaria, but in practice they're not well developed aids for navigation. Instead sailors in the Adelantean and Sea of Salaah tend to rely on a combination of portolan charts with rhumb lines, astronomical observations, and periplus-type navigational guidebooks to make their way.
Generally speaking navigation is at a much lower level of sophistication than Earth-navigation was at the same Renaissance-ish time period, because Avarian sailors aren't engaged in long-distance oceanic travel at this point. Adelantean sailors stay close to the coastline because trying to sail across is courting almost certain disaster, and the seasonal trade routes around the Adelantean basin are fairly predictable (even if the weather and currents aren't necessarily). In the Sea of Salaah there is more open-ocean sailing, but islands and landmarks are fairly plentiful, so that it's relatively easy to stay on course without more advanced "scientific" aids. Sailors in the Salaah have extremely honed their skill of sailing by the stars, though, and use the rising and setting of the stars and the position of the moon (especially Celeiri) within the different "houses of the moon" to chart their way very effectively.
This topic makes me really want to go and revisit the Idiri map (my oldest map) and fix it up, maybe even with equatorial and madaar lines!
