This may seem like a silly question, but it is something we see all the time among STK users, especially when they are working with other tools that also have their definition of the Earth. STK has a default way to define our home planet and it is important to understand that definition as well as how you can change the world (in STK at least).
The default representation of Earth is the WGS84 ellipsoid. This means that the Earth is an oblate spheroid based on the parameters specified in the WGS84 definition. More information about the WGS84 definition and how to edit this reference ellipsoid can be found in this article.
Just as a circle is a special case of an ellipse, a sphere is also a special case of an ellipsoid. By setting the minor-axis of the Earth ellipsoid equal to the major-axis, you can change STK’s Earth to a perfect sphere rather than an oblate spheroid. This article goes into more detail about that, as well as other simplifications you can make to the Earth definition.
In reality, the earth is neither an ellipsoid or a sphere. To truly represent the Earth, you must include terrain, which can create variations in the surface of the Earth both above and below the reference surface. STK has a built-in terrain server if you are connected to the internet. You can also add your own terrain by following the steps in this tutorial.
Last but not least, there is the ever popular flat Earth. If you’re a believer or just a fan of pranks, check out this April Fools’ Day post.