This is the broadest possible overview for those who are unfamiliar with how the motions of the Earth fit in with human timekeeping and although it is an injustice to those people who created the system we use today, the overview is much better than none at all. There is no stopping to differentiate between the original geocentric astronomers and their observations from the later heliocentric astronomers who worked off the idea that the Earth moves between Venus and Mars around the Sun so readers today don't concern themselves with the level of knowledge of any given era as this will be taken for granted in order to move the story along.

As the Earth moves through space in its circuit around the Sun,certain stars lying on or near the same plane as the Earth's orbital motion are temporarily lost behind the Sun's glare. The older astronomers marked the passage as the Sun through the constellations (hence birth signs) but for purposes of this story it is better to consider the apparent motion of the stars behind the Sun due to the orbital motion of the Earth -

https://www.youtube.com/watch?v=eeQwYrfmvoQ

The anchor for the Lat/Long system originally was based on the appearance of one particular star as the orbital motion of the Earth placed the star just far enough to one side of the Sun one morning, That star was the brightest one in the celestial arena known as Sirius and can be seen on the bottom left of the image -

https://www.youtube.com/watch?v=eeQwYrfmvoQ

".. on account of the procession of the rising of Sirius by one day in the course of 4 years,.. therefore it shall be, that the year of 360 days and the 5 days added to their end, so one day shall be from this day after every 4 years added to the 5 epagomenae before the new year" Canopus Decree 238 BC

Unbeknownst to themselves, what the Egyptians were actually doing was defining the Earth's orbital position around the Sun using the seasonal appearance of a star as a gauge and the number of rotations that fit inside an orbital circuit, in this case 1461 rotations for 4 orbital circuits which breaks down into 365 1/4 rotations to one orbital circuit.

This is the founding principle where one 24 hour day keeps in step with one rotation of the Earth and the basis of the Lat/Long system along with the reason why the extra day and rotation is introduced in order to keep the Earth's position in space fixed by the number of rotations,at least to the nearest rotation.

The next step is the process which equates the 'average' 24 hour day with 'constant' rotation insofar as the average 24 hour day substitutes for constant rotation through 360 degrees in that period.