Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis.

This could have weakened the greenhouse effect and cooled the Earth so much that the tilt towards the sun every 41,000 years no longer provided enough heat to melt the glaciers that formed in between.

In ideal conditions, the effect of a tilt or a cave is exactly opposite for the two transmitters, so that an average gives the correct result.