Orbits of the Moon and Planets
In Texas, teachers are expected to teach that the planets orbit the Sun, and that the Moon orbits the Earth. This topic is closely related to the changes in the night sky, the position of the planets, and lunar phases.
3.11 C The student is expected to identify the planets in our solar system and their position in relation to the Sun
4.6 A The student is expected to identify patterns of change such as in weather, metamorphosis, and objects in the sky
5.12 D The student is expected to identify gravity as the force that keeps planets in orbit around the Sun and the moon in orbit around the Earth
7.13 B The student is expected to relate the Earth's movement and the moon's orbit to the observed cyclical phases of the moon
This is a fairly simple topic, but is related to others that are much more complex (seasons, lunar phases) and so if students do not understand the movement of planets about the Sun or the Moon around the Earth, they will be unable to truly understand these other concepts.
In order to understand the orbits of the planets around the Sun, students need a basic definition of planet and Sun or learn some of the differences between the Sun and the planets.
In order to understand details about the planets' elliptical orbits, students need to understand the difference between a circle and an ellipse.
In order to understand why the planets orbit the Sun, students need to understand the properties of gravity.