Direction, Latitude and Longitude 1 — Page 14, Lesson 24

Let’s pick up right where the direction and longitude ideas start to get tricky. I want to walk you through something that genuinely trips up a lot of students: the reversal of the apparent sense of longitude at the Greenwich anti-meridian, which is the 180°E longitude. Here’s the setup. Imagine you’re standing on the Greenwich Meridian, the 0° line of longitude. East is to your right, West is to your left. The Eastern Longitudes are East of you, and the Western Longitudes are West of you. That feels natural. Now travel round the globe to the 180°E longitude — that’s the Greenwich anti-meridian, exactly halfway around the world from Greenwich. Here’s the key point: the direction of East has not changed. East is still 090°(T), which is the direction of the Earth’s spin. And West has not changed either — it’s still 270°(T), the opposite direction to the Earth’s spin. So compass-wise, nothing flips. But here’s the confusion. At that anti-meridian, the Eastern Longitudes are now out to your left — that’s West of you. And the Western Longitudes are out to your right — that’s East of you. So you get a situation where the Eastern hemisphere is to your West, and the Western hemisphere is to your East. It’s a complete reversal of what you’d expect, and it can cause real confusion when you’re solving navigation problems. Just remember: the cardinal directions themselves never change, but the geographic hemispheres appear swapped from that vantage point. Now let’s move to a fundamental difference in principle between latitude and longitude. These are two angular coordinate systems, and they work quite differently. Lines of latitude are all parallel to each other — that’s why we call them ‘parallels of latitude’. They never converge. Lines of longitude, however, are a different story. They emanate from a point — the North Pole — reach a maximum separation at the Equator, and then converge back to a point again at the South Pole. So meridians of longitude are not parallel; they pinch together at the poles. A helpful illustration: think of latitude as like slicing a pineapple — you get parallel rings. Longitude is like segmenting an orange — the segments all meet at the top and bottom points. That’s the mental picture. Now, how do we actually write positions? The rule is fixed: latitude is always quoted first, longitude second. So New York’s position is 41°N 074°W. There are alternative forms. You can write 41°00’N 074°00’W, or you can write 4100N 07400W — those forms drop the degree symbol and use minutes of arc to allow more precision. The minutes are the ‘00’ part. Here’s a real example. The Aerodrome Reference Point — the ARP — for Oxford Airport is approximately 51°50’N, 001°19’W, or in the compact form 5150N 00119W. That ARP is the mid-point of runway 01/19. And if you need even greater precision, you can quote seconds of arc. That’s not normally required in these notes, but for example, the precise position for the Oxford ARP is 51°50’12”N, 001°19’11”W, or in the compact form 515012N 0011911W. So you see the pattern: degrees, then minutes, then seconds, latitude first, longitude second. Let me just make sure the key contrasts are clear. Latitude lines are parallel; longitude lines converge. Latitude is quoted first; longitude second. And at the anti-meridian, the hemispheres appear reversed even though East and West themselves don’t change. That’s the core of this section.