2,665. An Oath About the Size of the Sun

Hilchos Shevuos 5:22

In the Rambam’s time, scholars knew that the sun was much larger than the Earth. Nevertheless, if a regular person took an oath that the sun is larger than the Earth, he would not be liable for making an oath in vain because this fact was not commonly known among the average populace. One is only liable for making a vain oath when he takes an oath about something that is known and obvious to three laypeople, such as that a man is a man or that a rock is a rock. Similarly, if someone took an oath that the sun is smaller than the Earth, he wouldn’t be liable even though such is not the case because the facts were not commonly known. This is not like taking an oath that a man is a woman because his oath was based on his perception of how the sun appears. The same is true in comparable cases based on the calendar, astronomy, geometry and other disciplines whose conclusions are not self-evident.

Hilchos Shevuos 6:1

Let’s say that someone took an oath of expression and later regretted it. If he anticipates that he will suffer if he fulfills the oath and changes his mind, or if something happened that he didn’t anticipate when taking the oath and this causes him to change his mind, he can appeal to one Torah scholar, or to three laypeople in a place that doesn’t have Torah scholars. His oath is permitted and he may then do the thing that he took an oath not to do, or not do the thing that he took an oath to do. This is called permitting oaths.