How many miles is it around the earth at the 45th parallel?

The Earth's circumference at the 45th parallel is approximately 16,042 miles (25,819 kilometers). This is calculated by multiplying the Earth's average circumference of about 24,901 miles (40,075 kilometers) by the cosine of 45 degrees, which is approximately 0.7071. This calculation assumes the Earth is a perfect sphere, which is a simplification as the Earth is actually an oblate spheroid.