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To get the index correction inland, you can use a distant object like the roofline of distant building, if the sextant is held vertically, or maybe a radio tower, if the sextant is horizontal. The nice thing about using a distant, vertical terrestrial object for an index correction is that you can set the sextant down on its side on a table. Then you can either accurately measure the index correction or adjust it out as you like, taking your time with no fatigue.

There is a small parallax within the instrument that you have to watch out for. The line of sight through the index mirror and the line of sight through the horizon glass are typically separated by about 3 inches. If you want the index correction accurate to the nearest minute of arc (rather low quality), then you need to make sure your target object is at least 3438*3 inches away from you, which works out to about 860 feet (see PS). If you want the index correction accurate to less than a TENTH of a minute of arc (the best you could hope for with a modern sextant), then the target object should be at least twenty times further away, which is about 3 miles or 5 km. By the way, this also applies to the sea horizon, but for any height of eye greater than about seven feet, that's guaranteed.

There are a couple of ways to use astronomical objects to do an index correction. For the moment, aligning one on top of the other is just fine. I recommend the planet Jupiter for this. It's easy to find in the western sky right now just after sunset, and its small disk eliminates the "flaring" issue that you may discover with bright stars.

-FER
PS: The factor 3438 is just 180*60/pi and converts any ratio of distances to an angle in minutes of arc.

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