In the movie Back to the Future, the concept of time travel is presented in a fictional and fantastical way, and it doesn't necessarily adhere to the laws of physics as we understand them. While the movie incorporates elements of time travel and the effects of changing the past, it doesn't delve into the specific mechanics of how the Earth's orbit or the movement of the solar system would impact time travel.
In reality, if someone were to travel forward or backward in time, it would indeed pose some challenges when considering the Earth's movement around the Sun. If time travel were possible, it would be expected that a time traveler would reappear at the same spatial coordinates on Earth, regardless of the planet's position in its orbit around the Sun. This is because when we talk about time travel, we generally assume that it is independent of the Earth's motion in space.
However, as Back to the Future is a work of fiction, it takes creative liberties and doesn't go into the details of how the time travel mechanism in the DeLorean specifically accounts for Earth's orbital motion. The focus of the movie is primarily on the narrative and the adventures of the characters rather than providing a scientifically accurate portrayal of time travel.
It's worth noting that time travel is purely speculative at this point, and there is no scientific consensus on its feasibility or the specific mechanisms involved.