Stick with me on this one, This is purely fictional, and curiosity.
Say we travel to another solar system we know nothing about. We land on another habitable planet. We don't know the mass of any planet around us or the one we landed on. We have no idea the mass of the sun. We also don't know the radius of any spherical body. We also don't know the distance between the planet we landed and the sun or any other planet for that matter.
In summary, we don't know anything.
How do we find the mass of the planet we are on?
How do we find the gravity of the planet?
How do we find the distance between the planet and the sun?
Is time our only friend here? How do we know when a full revolution around the sun has taken place? what if it takes 100 years for a full revolution?