To act as Earth's moon, Mars would need to be at a distance similar to that of the Moon from Earth. The average distance between the Earth and the Moon is about 384,400 kilometers (238,900 miles). Mars, on the other hand, has an average distance from Earth of about 225 million kilometers (140 million miles) when the two planets are at their closest approach (opposition).
Therefore, Mars would need to be significantly closer to Earth than its average distance in order to appear similar in size and proximity to the Moon. However, it's important to note that even if Mars were somehow positioned at that distance, it would still have several observable differences compared to the Moon, including its reddish hue, larger size, and different surface features.