In the Cambridge Star Atlas 2000.0, two of the horizon line labels are flipped on one side of the page. Typo, or is there a reason?

Question

I recently acquired the Cambridge Star Atlas 2000.0, and noticed something odd when taking it out observing. On the north side of the month-to-month star charts, the Mercator latitude indicators at the 20 degree marks appear flipped on the right hand side - what seems like it should be 20 deg N is listed as 20 deg S, and vice versa.

To my best understanding, this must be a typo - I can't imagine why I'd need the horizon to be diagonal on this map, and it seems like it would break the 'circle' of the horizon to draw a line from N-N or S-S as listed on this image. Still, I want to confirm: is this a typo?

Answer 1

This is a typo. The 20N and 20S indicators circled are flipped.

What follows is some intuition as to why we can know this is a typo at a glance:

Look carefully at the 20S lines. If you draw a line from the listed 20S point on the left, to the listed 20S point on the right, Polaris would be visible well below the equator, all year long. Since Polaris is almost aligned true North (with a declination of 89.3 degrees), this should not be possible, except when very close to the equator with optimal viewing conditions.

To give more intuition, imagine yourself starting at 60N and moving down the globe. As you move, Polaris would change position in the sky - starting high, and falling with your altitude. But once you hit 20N, for this to not be a typo, Polaris would have to move suddenly to the horizon, stay there until 10N, when it jumps upwards again, and then track its way down below the horizon.