For vacation, you decide to take a solo boat trip. While contemplating the unive

a.  You are on the equator. Because it is the summer solstice, the Sun crosses the meridian 23.5° north of the celestial equator. Thus the Sun's meridian altitude of 67.5°N tells you that the celestial equator is passing through your zenith, and hence that you are on the earth's equator.

b.  Your longitude is 90°E. The Sun is on your meridian, so it is noon for you. The UT clock reads 06:00, or 6 A.M., so Greenwich is 6 hours behind you. Each hour represents 15° of longitude, so 6 hours means 90°; you are east of Greenwich because your time is ahead.