Sky Clock

understand what the hours on the clock are about

Does time repeat over and over again? This is what clocks would suggest, generally returning to the same state every 12 hours. Nobody is confused, but software allows us to improve on this. So I took the opportunity to fix what's not broken :), with a spiral based clock. But there is more to it...

The day's hour is a shorthand for how far along we are within one day-night cycle — between the last time the sun was, and the next time it will be, at the highest point in the sky. During this time, the lighting, the color of the sky, and Sun's apparent position change roughly the same way every day. That's the most natural association children can form for the hours on the clock, and is usually the first. The Sky Clock gadget can help with that.

The hour-sky model represented here is an approximation. Have fun finding incorrect implications by this gadget — some of them mentioned in the Dig Deeper section. (Share yours in a comment.)

Like an actual clock, this gadget technically returns to the same state, too, "after" 24 hours. But the spiral creates a different perception in one's mind: that new hour numbers — points in time — continue to arrive from somewhere outside of the dial and, once they move towards the center, we never see the same one again. As long as we move the time forward. :)

cris p

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This is a simple view that reinforces the days' hours' succession and the relations to the apparent sun position — move the clock hand to see how things change. Children can try this before going to the Clock Time page to refine the understanding of time on the clock.


number spiral
The positioning of the hours on a spiral says that time doesn't repeat after completing a cycle — what repeats is the labels that we use for the hours. Time is "infinite" in both directions.
The background color is roughly the sky color at the indicated hour. Separately, AM and PM are indicated with their own colors — yellow and cyan.
The background represents the sky. The sun and stars are coordinated with the current time: stars fade in and out; the sun moves along an ellipse. The sun stays visible, but fades out as it descends below the horizon.
AM and PM are coded with the yellow and cyan colors. They are used in several places.

An AM/PM boundary line divides the picture into two halves: it's AM when the sun is in one half and PM when it's in the other.

The AM/PM boundaries are represented on the clock dial, too. There are two AM/PM boundaries. Depending on DST, they can coincide, or be different — see the details next.

Daylight Saving (Summer) Time — DST
You can visualize the turning forward or back of the clock (by 1 hour) when the DST begins and ends. The relation between hours and sky changes depending on DST.

The AM/PM boundaries change, too. When DST is in effect, the sun is no longer at its highest point at 12 o'clock, but at 1. The gadget will now show two AM/PM boundaries, if enabled: the "nominal" one, at 12 (dashed line, yellow-cyan); and the "real" one, at 1pm, when the sun is at its highest point (solid, thick line, yellow on one side, cyan on the other).

Each time the view is reloaded, the drawings change. Children may want to refresh, to see what the new shape of the sun is, how tall the buildings are, the new star count, colors, sizes. (In most browsers, push F5 on the keyboard.)


Controls the display of the AM/PM boundary lines. It's AM when the sun is on one side of the line, PM on the other. The boundaries are shown on the clock, too.
This option changes the clock time (the position of the clock hand) by 1 hour, while keeping the "sky time" the same. Turning it on shifts the coloring of the spiral so that it's no longer symmetric with respect to 12 o'clock; two AM/PM boundaries are shown, because 12 o'clock no longer corresponds to the sun's highest position.
24 hours
Controls whether one day-night cycle has two 1-12 hour ranges or one 0-24 hour range.
sparse / packed
When "sparse" is checked, the spiral step is large, so that few hour numbers are visible at the same time. This is expected to be less distracting for the beginner.

Checking "packed" sets the spiral step to its minimum value. It makes it easier to see that two consecutive occurrences of the same hour number are different instances, associated with a different time of day — AM vs PM.

It is possible to have both unchecked, to get a spiral step that's in the middle.
N / S
Set the point of view in the Northern or the Southern Hemisphere. This affects the direction of sun's apparent motion.

Dig Deeper

What do AM and PM stand for?

Dictionaries tell us that they mean ante meridiem and post meridiem: before and after midday in Latin. And the middle of the daylight period is the moment when the sun is at its highest position in the sky. So AM and PM are connected to sun's apparent movement.

Why do we divide a day into AM and PM halves?

Here is my theory . . .
Thousands of years ago, the moment when the sun is at the highest point in the sky was important to people for various reasons. This provided a sensible anchor for defining time, but by itself not a reason to split one day's cycle into 2; however, it split people's waking hours in two parts (probably of similar lengths at the time).

When they needed to express the time of an event with more granularity than the day it happened, the next natural step was to place it within the period when "the sun is moving up" (AM) or "the sun is moving down" (PM). Later, it was more convenient to make clocks with half-day cycles - an answer to the opposing constraints of precision and simplicity.

Find all the occurrences of colors yellow and cyan, for AM and PM.

  • Nominal AM/PM boundary line - on the dial and in the picture
  • Real AM/PM boundary line - on the dial and in the picture
  • Hour numbers
  • AM, PM labels
  • The inside part of the clock dial

Does the sun reach the same height every day?

No, it goes higher in the summer, not so much in the winter.

Does the sun reach the same height everywhere in the world on a given day?

No, it's highest (sun directly above the head) on some line (that changes) parallel (almost!) and close to the Equator; and never rises above the horizon in some places close to the opposite Pole. The nearest Pole doesn't have night around that time.

Does the sun cross the horizon in the same places every day?

No, the span between the crossing points is larger in the summer and smaller in winter, changing gradually from maximum to minimum and back.

What's the minimum and maximum horizontal span between the points where the sun crosses the horizon?

It depends on the distance from the Equator. The minimum and maximum are reached in places beyond the Polar Circles: the minimum can shrink to nothing — when there is no day, and the sun stays below the horizon; and the maximum can grow to the whole span of the horizon, 360 degrees — when there is no night, and the sun stays above the horizon.

Why are these movements so complicated? How can somebody possibly remember all of this? Why does it matter?

Good questions! Answers are coming . . .   ;)

(See also the next question.)

Why does the sun move?

Sun's apparent movement in the sky is NOT the result of its actually moving. That movement is explained by the Earth's turning around an axis that goes through the Poles, while rotating around the Sun.

It's in part the same effect that happens when you take a video of the landscape while spinning around. When you watch the landscape directly (when spinning), you consider it fixed, knowing that you are the one moving; when you watch the video, you consider the screen fixed, so the landscape appears to move relative to the screen's edges.

In the past, people were fooled by the appearance of Earth being fixed: our everyday experience tells us that it's not moving under our feet; so it was natural to conclude that it's the Sun that's moving.

(The Sun does actually move a "tiny" little bit — it wobbles; do a web search for "sun wobble". This makes no difference for how we see it move in the sky.)

Further questions (even deeeeper stuff) . . .
Is our "immobile Earth" experience wrong? (Yes.) why? Is there an absolute fixed point in the Universe? if yes, where? if not, is all movement not a matter of interpretation or point of view? Could we (by definition) choose the Earth as the fixed reference? If we were to choose a reference, do we need just a point (e.g. center of Earth), or a solid (the whole Earth - with its 3D directions) as a reference? Is there a difference between choosing either of two "references", whose movement relative to each other is not constant, or not a straight line?

Is this really how the sun moves in the sky?

The positions and movement are not accurate - they are only meant to capture the basic idea. But I hope young users of the gadget will find some of it suspicious and will ask questions. :)

- - -

If I wanted to be more accurate, there is still the problem of projection - how to position on a 2D screen objects from a 3D space. There are different possibilities, some of which are more "natural" than others. So maybe I could claim that my gadget is accurate with respect to some complicated projection that would be far from what would be considered natural.  ;)

During the day, the sun moves from left to right or right to left, depending on whether we watch from the Northern or the Southern hemisphere. Why? Is this always so?

Imagine a line painted on the ground and a bird flying along it, exactly above it. People watching from different sides of the line will see the bird flying from left to right or from right to left, depending on the side they're on. In our case, the bird is the Sun and the line is the Equator.

This is if we approximate the Sun as being always above the Equator, which (almost) happens on equinox days. In reality you can be in the Northern hemisphere, close enough to the Equator, and see the sun moving from right to left — when it's summer in the North.

(With DST off) Is 12 o'clock when the sun is exactly at the highest point?

For most places, no. The clock time is the same within a time zone. But in different places within a time zone (other than on the same meridian), the sun appears to be at its highest point at different times. There is at most one meridian in a time zone where the sun is highest precisely at 12 o'clock. In other places, there is a variable difference from 12, usually less than half hour; but it can exceed one or more hours, too.

Why do many places use Daylight Saving Time?

For an answer, put these ideas together . . .

* Most people's schedules are these days driven by the clock, not by daylight, and they are relatively consistent in relation to the clock, but not the sun.

* The sleep period is generally asymmetric with respect to 12am (midnight): we sleep at least twice more after 12 compared to before.

* During the summer, the night can be quite short (shorter closer to the Poles).

* We prefer to sleep when it's night and do most activities during daytime.

In reality, I never see the sun at night. Why does the gadget show it below the horizon?

The sun is intentionally left visible when below the horizon, to suggest that it's still there, somewhere, we just don't see it from the location where it's nighttime.

If the Earth were transparent, would we see the sun in the position depicted here?

Usually, no — the apparent/virtual trajectories above and below the horizon are in general not symmetric.

Why are there no stars during daytime?

The stars continue to exist during daytime. The earth blocks the view for many of them — they are below the horizon. But we don't see those above the horizon either ; that is because their faint light is overwhelmed by Sun's light. (In this gadget, this is also the reason why they are not shown below the horizon: the brightness of the sky extends there, too.)

Isn't the sun supposed to be round? Why do we sometimes draw it with "rays" of different shapes?

The Sun IS round (allowing for some imprecision). I think we draw it with rays in order to distinguish it from the Moon. Or some random ball flying in the air :).

I hope you enjoy the slight unpredictability of the shapes produced by the gadget.

Why do I sometimes spell "sun" and other times "Sun" (capitalized)?

When I talk about the way we see it in the sky, I write "sun". When I talk about the huge "ball of fire" that we move around, I write "Sun".

In other words, "sun" is kind of 2D and "Sun" is 3D.

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