ATPL · Navigation · Oxford Atpl Book General Navigation 6th… lesson — Page 387, Lesson 340

Let's start with the very foundation of timekeeping in navigation: the day. A day is defined as the length of time it takes for the Earth to rotate once about its axis, measured against a celestial body — either the Sun or a star. That choice of reference body gives us two different kinds of day. Measurements against a star are called 'sidereal', and measurements against the Sun are called 'solar'. Keep those two words in mind — sidereal for stars, solar for the Sun — because everything else in this chapter builds on that distinction. Now, before we dig into the technical definitions, let's look at what a 'civil' day should be. A civil day is the day we actually use for everyday life, and it has two requirements. First, it should be related to periods of light and darkness, so that 1200 hours — noon — is always about halfway between sunrise and sunset. That means the civil day must be based on the Sun, because the Sun is what drives our light and dark. Second, a civil day must be of a constant length. So we have two criteria: tied to the Sun, and constant in duration. Let's test the sidereal day against those criteria. A sidereal day is measured against a distant star, and it is of nearly constant length. That's good — it satisfies the constancy requirement. But here's the problem: it is not related to light and dark. Because it's measured against a star, not the Sun, it drifts relative to sunrise and sunset. So a sidereal day is not suitable as a civil day. It's constant, but it doesn't keep noon halfway between sunrise and sunset. Now let's look at the apparent solar day. This is measured against the real or apparent Sun — the one that actually appears in the sky to you. And here we run into the opposite problem. Using the apparent Sun introduces the issue that the apparent solar day is not a constant length. So we have one candidate that's constant but not Sun-related, and another that's Sun-related but not constant. Neither alone works as a civil day. To understand why the apparent solar day isn't constant, we need to consider the Earth's orbit around the Sun. Let me set up the geometry. Imagine viewing the Earth's orbit from the North Celestial Pole, or NCP. The NCP is an imaginary point in space, located along the continuation of the Earth's axis — that is, you take the line running from the South pole through the North pole, and project it out into space. That's the North Celestial Pole. Now, consider the Earth at position A in its orbit, but imagine a false situation where the Earth is stationary — not moving along its orbit. An observer at position Z on the Earth's surface would have the Sun and a distant star directly over his meridian at the same moment. After one complete anticlockwise rotation of the Earth, both the Sun and the star would again be over the observer's meridian. In this stationary case, the apparent solar day and the sidereal day would be equal. But this is a false situation — the Earth is not stationary. Here's the real situation. In the period of one full 360° revolution, the Earth travels along its orbit from position A to position B. After that 360° rotation, the distant star is again over the observer's meridian — that's one sidereal day. But the Sun is not yet back over the meridian. Because the Earth has moved along its orbit, the observer needs an additional rotation, and the Earth needs to travel further along its orbit to position C, before the Sun is again over the observer's meridian. So the apparent solar day — measured against the Sun — is longer than the sidereal day, measured against the star. That's the key relationship: an apparent solar day is longer than a sidereal day. But remember, the Earth's orbital speed changes throughout the year. The Earth doesn't orbit the Sun at a constant speed. So the amount of extra rotation needed to catch up with the Sun varies from day to day. And that means the apparent solar day cannot be of constant length. The length of the apparent solar day fluctuates through the year. So how do we get a civil day that satisfies both requirements? That's where the mean solar day comes in. The mean solar day is the average length of an apparent solar day, averaged over the year. Because it's an average, it is of constant length. And because it's based on the Sun, it is related to light and darkness. So the mean solar day is used as the civil day, and it is divided into hours, minutes, and seconds of 'mean' time. Here's a helpful way to think about it. Sometimes it's easier to imagine the Sun travelling westwards around the Earth, rather than the Earth spinning eastwards. In the case of mean time, we consider the mean — or average — Sun circling the Earth every 24 hours. This is the basis of Local Mean Time, or LMT. So when we talk about mean time, we're imagining a fictitious average Sun that moves at a perfectly uniform rate, completing one circuit every 24 hours. Now, because the real Sun doesn't move uniformly, there's a difference between mean time and apparent — real — Sun time. The maximum difference between them is about 16 minutes, and it occurs in mid-November. There's a second maximum of about 14 minutes, occurring in mid-February. In between these maxima, the difference reduces. This difference between mean time and apparent solar time is known as the Equation of Time. So let me tie this all together. We have three kinds of day. The sidereal day, measured against a star, is nearly constant but not tied to light and dark. The apparent solar day, measured against the real Sun, is tied to light and dark but not constant, because the Earth's orbital speed varies. And the mean solar day — the average of the apparent solar day — is both constant and Sun-related, which is why it's our civil day, divided into mean hours, minutes, and seconds, and forming the basis of Local Mean Time. The Equation of Time is simply the name for the difference between that mean time and the real Sun's time, peaking at about 16 minutes in mid-November and about 14 minutes in mid-February.