The gang is attempting to find the planet Mercury through a telescope, which is pointed at a very high angle. Mercury, even at its greatest elongation, is always close to the horizon and could never be seen at such an angle.
The gang is said to be on the north side of the roof, trying to see planets on Raj's telescope, which would thus appear to be pointed northward. However, in the northern hemisphere, the planets would all be visible in the southern part of the sky.