How a sundial tells the time

The Earth turns, so the sun appears to travel across the sky, and shadows swing. A stick in the ground is already a clock of a rough kind. A sundial is that stick, set at the right angle, with the hours marked where the shadow will fall.

The gnomon is not usually upright. In a common garden dial it points at the celestial pole, which means it leans at an angle equal to the latitude of the place. Set that way, the shadow’s direction, not its length, tells the hour, and the same marks work through the year.

A shadow and a rod

The marks are not always evenly spaced, especially on a flat horizontal dial, because the shadow’s movement projects onto a flat plate. A vertical dial on a south-facing wall has a different set of spaces. The maker draws them for that face and that latitude. A dial made for London is a little off in Edinburgh.

Sundial time and watch time differ for two reasons. The Earth’s orbit is not a perfect circle and the planet is tilted, so the sun runs fast or slow against a steady clock by up to a few minutes either way. That difference is the equation of time. On top of it, civil time is a zone, the same across a wide area, while a dial shows the local sun.

Why it disagrees with a watch

A dial does nothing at night, nothing in thick cloud, and nothing useful indoors. It was a public timekeeper when a tower was the clock most people had, and it remains an honest one: it tells you where the sun is, which is what the hours originally were.

If you set one, set the gnomon to true north, not magnetic north, at the latitude you are in. Then it will agree with the sun. It was never obliged to agree with the radio clock.

A shadow and a rod

A sundial uses the shadow of a rod, the gnomon, to show where the sun is in the sky. The marks are hours. The sun does the moving. The Earth turns, so the sun appears to travel across the sky, and shadows swing. A stick in the ground is already a clock of a rough kind.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a sundial tells the time needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

Why it disagrees with a watch

A stick in the ground is already a clock of a rough kind. A sundial is that stick, set at the right angle, with the hours marked where the shadow will fall.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

A sundial is that stick, set at the right angle, with the hours marked where the shadow will fall. The gnomon is not usually upright.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

The gnomon is not usually upright. In a common garden dial it points at the celestial pole, which means it leans at an angle equal to the latitude of the place.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

In a common garden dial it points at the celestial pole, which means it leans at an angle equal to the latitude of the place. Set that way, the shadow’s direction, not its length, tells the hour, and the same marks work through the year.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

Set that way, the shadow’s direction, not its length, tells the hour, and the same marks work through the year. The marks are not always evenly spaced, especially on a flat horizontal dial, because the shadow’s movement projects onto a flat plate.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

The marks are not always evenly spaced, especially on a flat horizontal dial, because the shadow’s movement projects onto a flat plate. A vertical dial on a south-facing wall has a different set of spaces.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

A vertical dial on a south-facing wall has a different set of spaces. The maker draws them for that face and that latitude.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

The maker draws them for that face and that latitude. A dial made for London is a little off in Edinburgh.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a sundial tells the time needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The result that follows

A dial made for London is a little off in Edinburgh. Sundial time and watch time differ for two reasons.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

Sundial time and watch time differ for two reasons. The Earth’s orbit is not a perfect circle and the planet is tilted, so the sun runs fast or slow against a steady clock by up to a few minutes either way.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

The Earth’s orbit is not a perfect circle and the planet is tilted, so the sun runs fast or slow against a steady clock by up to a few minutes either way. That difference is the equation of time.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

That difference is the equation of time. On top of it, civil time is a zone, the same across a wide area, while a dial shows the local sun.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

On top of it, civil time is a zone, the same across a wide area, while a dial shows the local sun. A dial does nothing at night, nothing in thick cloud, and nothing useful indoors.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

A dial does nothing at night, nothing in thick cloud, and nothing useful indoors. It was a public timekeeper when a tower was the clock most people had, and it remains an honest one: it tells you where the sun is, which is what the hours originally were.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

It was a public timekeeper when a tower was the clock most people had, and it remains an honest one: it tells you where the sun is, which is what the hours originally were. If you set one, set the gnomon to true north, not magnetic north, at the latitude you are in.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

If you set one, set the gnomon to true north, not magnetic north, at the latitude you are in. Then it will agree with the sun.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a sundial tells the time needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The result that follows

Then it will agree with the sun. It was never obliged to agree with the radio clock.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

The Earth turns, so the sun appears to travel across the sky, and shadows swing. A stick in the ground is already a clock of a rough kind.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

A stick in the ground is already a clock of a rough kind. A sundial is that stick, set at the right angle, with the hours marked where the shadow will fall.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

A sundial is that stick, set at the right angle, with the hours marked where the shadow will fall. The gnomon is not usually upright.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

The gnomon is not usually upright. In a common garden dial it points at the celestial pole, which means it leans at an angle equal to the latitude of the place.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

In a common garden dial it points at the celestial pole, which means it leans at an angle equal to the latitude of the place. Set that way, the shadow’s direction, not its length, tells the hour, and the same marks work through the year.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

Set that way, the shadow’s direction, not its length, tells the hour, and the same marks work through the year. The marks are not always evenly spaced, especially on a flat horizontal dial, because the shadow’s movement projects onto a flat plate.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.