The Sextant is a great little tool with two different functions for the basic variety (you will be able to craft an Augmented one later).
At night with the Sextant in hand, right-click and you will notice that it looks a lot like the Looking Glass, but with a few extra icons on the right hand side (each icon represents different astral Sorcery Shrines: Large, Desert and Small).
As you look around the night sky with the Sextant you will notice large coloured stars (sometimes blue, sometimes green). These stars are locations of Astral Sorcery Shrines. While looking at the star you can hold SNEAK. This enables you to move your mouse without looking around the sky. If you hover over one of these stars you will be able to see what kind of Shrine is nearby.
You can then drag and drop the correct kind of Shrine from the icons on the right. This will make the star appear in the night sky without using the Sextant, thus making it far easier to find Shrines. Once you have found the Shrine you want, you can select another star in the sky and this will stop tracking the previous Shrine. Green stars are undiscovered and Light Blue are discovered shrines.
The Sextant is not just used for finding Shrines. You can also use it to assist you with building the required structures around Altars (such as the Crafting Altar). Once you have upgraded your Altar, you can sneak-click the Altar with your Sextant. This will show “ghost blocks” where you need place down blocks in order to complete the Altar.
This way you can see what blocks to use and exactly how high and wide to build the structure. This makes building much quicker. This can also be done with other structures in the future but you MUST have the base block in place. This does not work with ALL structures (such as relays).
In creative mode, sneak-right-clicking will place down all the blocks for you.