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SYFY WIRE Bad Astronomy

Noodling Google’s Doodle

By Phil Plait

I like Google. I know, I know, there have been some issues with them, and I understand all that. But the company really does seem to try to make the world a better place as well as it can, and while there have been some stumbles, a lot of what Google does is really wonderful.

Iâm also a big fan of small wonders; just little things that make life a wee bit more fun. Thatâs why I like the Google Doodlesâdrawings or animations they put at the top of their search page, usually related to the day itâs up. For example, today is Earth Day, so they have a terrific little animated cartoon showing the sun and moon moving across the sky over the Earth:

The Doodle is adorable, showing fish swimming, water and air circulating, and even prairie dogs running around (at least, I assume theyâre prairie dogs; those cute varmints are all over Boulder, so maybe Iâm biased). And since itâs a celebration of Earth Day and all the bounty our planet has to offer, it would take an abominably curmudgeonly anal-retentive jerk to notice that perhaps, just maybe, there might be a few scientific errors in the Doodle.


OK, mea culpa. I canât help it. I do like this Doodle, but itâs like an itch I have to scratch: There are a few mistakes in it. They arenât a big deal, but neither is that tiny little itch located just perfectly in the small of your back where you canât reach it and you have to scramble all over the house looking for something sticking out you can rub up against to scratch it.

So here are some of the scientific boo-boos in the Doodle. And before you send me hate mail, please read the last section of this articleâI may be a little curmudgeonly, but my heartâs in the right place.

The phase of the moon is shown the wrong way.

As the Doodle cycles, you see the moon rising on the left and setting on the right (which is correct for someone in the Northern Hemisphere facing south; east is to the left and west to the right). The first time we see the moon, itâs a crescent rising in the east at sunset, oriented with the wide part to the left, and the horns of the crescent pointing to the right.

But thatâs not possible. When the moon is opposite the sun in the sky, it has to be full. Hereâs why.

The reason we see phases of the moon is due to the geometry among the Earth, moon, and sun, which changes as the moon orbits the Earth. When the sun and moon are in the same part of the sky, the moon is new. A few days later, as the moon circles the Earth, it pulls away from the sun in the sky, and we see a crescent, with only part of it lit. A few days more (a week after new moon), and the moon is half-lit (what we call, weirdly, first quarter, because itâs a quarter of the way through its monthly cycle). A few more days, and the moon gets fatter, and has whatâs called a gibbous shape. Then, two weeks after new moon, itâs opposite the sun in the sky and we see it as full, a completely lit disk.

After that, the cycle reverse. The moon becomes gibbous, then half-lit, then a crescent again. Since itâs at the end of its cycle, we call that the old moon.

You can see all this in an animation put together by the folks at NASAâs Goddard Space Flight Center:

For some reason, the Google Doodle starts with the old moon. Thatâs fine, but the way itâs depicted is incorrect: The crescent moon has to be near the Sun in the sky. Thatâs why itâs a crescent. Itâs shown as opposite the Sun, rising in the east as the Sun sets in the west, which only happens when the moon is full.

The phases are out of order.

So the moonâs phases go through a cycle once per month, which is how long it takes the moon to orbit the Earth (and is where the word month comes from; think moonth).

So it starts new, is then a thin crescent, a fatter crescent, half-full, gibbous, then full. After that the lit portion shrinks, going through gibbous, half-lit, then a crescent again (the old moon). Lather, rinse, repeat.

As I pointed out, the Doodle starts with the old moon (the horns point to the right).* But the next phase we see is the gibbous moon. Thatâs not correct; the next phase should be the new moon.

The old moon is nearly aligned with the sun (on the right of the sun from the Northern Hemisphere). As the moon orbits the Earth a bit more, it gets nearer the sun, then starts to pull away to the left. So the next phase after the old moon is actually the new moon, a thin crescent with the horns pointing to the left.

Iâll note that this gets a bit more complicated, because the Earth is a ball, too. Travel south, to the Southern Hemisphere, and things get reversed because youâre upside-down compared with the Northern Hemisphere. The new moon will have the horns pointing to the right, not left, as you face the setting sun. So in that sense, some of the Doodle might be saved, because now the animation starts with the new moon, not the old one, and the next phase would be half-lit, then gibbous. But even then, the crescent moon is still in the wrong part of the sky. Worse, the motion of the sun, moon, and stars would be right to left, not left to right. So in either hemisphere the Doodle wonât work.

Did I mention you have to be anal-retentive to spot all this? Yeah.

The stars donât move.

We see the moon move in the Doodle as it rises and sets, but the stars are stationary. In reality they all rise and set, which is really just a reflection of the Earth spinning on its axis.

As it happens, because the moon is orbiting the Earth, it doesnât move at the same speed as the stars in the sky. On top of the motion due to the Earthâs spin (called diurnal motion), which makes the moon, sun, and stars move east to west, the moon is moving slowly to the east. That means it moves a little bit slower than the stars. Over the course of the night, itâs barely noticeable, but itâs enough to cause the moon to rise about an hour later every day.

So in the Doodle, the stars and moon should be moving almost exactly together.

The dark part of the moon is transparent.

This one always cracks me up. We see a crescent moon because itâs a sphere, and only part of it to the side is lit by the sun. You can see this for yourself pretty easily: Go outside on a sunny day with a ping-pong ball (or some other sphere). Hold it up near the sun (donât look directly at the sun, please!), and youâll see the ball is lit just like a crescent moon. Rotate yourself so the sphere moves farther away from the sun, and more of it will be lit, mimicking the moonâs phases.

But the dark part of the moon is just unlit landscape; itâs still part of the solid moon. So when the moon passes in front of stars, it blocks those stars, and it doesnât matter if the part blocking it is lit or not. You canât see the stars through the solid (and very, very opaque) moon.

In the Doodle, you can see stars right through the unlit part of the moon, which is pretty common in cartoons and drawings. This could only happen if the moon were transparent, like made of glass or crystal.

Which, to be fair, would be totally cool.

The rising and setting sun (and moon) speed up.

This is just a nitpickâwell, all of this is, but itâs funâbut in the Doodle, as the sun rises itâs moving faster than when itâs high in the sky, and then speeds up as it sets, too. Same with the moon. In reality, the motion of the sun, moon, and stars is constant throughout the night. Remember, the motion of the objects in the sky is actually just due to the Earth spinning, which it does at a constant rate. So the motion of those objects is constant, too.

Though, to get really super-anal (a superpower to use very, very sparingly), the Earthâs atmosphere screws that up a bit. It acts like a lens, bending light. This bending (called refraction) is greatest when an object is near the horizon. But it goes the opposite way than shown in the Doodle: It actually slows down the apparent motion of the rising and setting sun (and moon).

Iâll note this is the same effect that causes the sun and moon to sometimes look flattened, squished, when theyâre on the horizon, and plays a part in why they look red on the horizon, too. Itâs a very cool and lovely effect, and one of my favorite things to see!

And one more that I know Iâll get mail about â¦

In the Doodle, the Earth is shown as being flat. It isnât. But then, itâs not a perfect sphere, either â¦

I know, I know. I only mention this out of completeness. If you think Iâm being anal, you canât even imagine the comments Iâd get if I left that part out. So there you go.

Dandy Doodle

Just to be clear, let me say again I really like this Doodle. It's adorable, and quirky, and fun. And, like I also said, all my points are nitpicky. Still, itâs fun to point them out, and maybe show you the way things really work.

And itâs more than just me seeing something wrong on the Internet. You have to remember: All of the things shown in the Doodle are actually happening above your head in the sky right now. The moon is ceaselessly circling the Earth, and the Earth is moving around the sun once per year as it has for eons. The motion of the celestial orbs is an amazing, graceful, and predictable dance. The laws of gravity and of motion are so well understood that we can launch probes from Earth to other worlds, and have them travel for many years and hundreds of millions of kilometers, and still thread an incredibly narrow needle to reach their targets. We can land a one-ton nuclear-powered laser-eyed rover on another freaking planet, and itâs because of science.

And all that knowledge gained, all that wonderful math and physics and engineering and exploration, it all starts because someone had the curiosity to look up, and the audacity to suppose that all that intricate motion must be due to some underlying rules.

Thatâs what looking up does. It shows you the whole universe. And honestly, if todayâs Google Doodle (and even my silly analysis) sparks someone to go outside and just look up, then mission accomplished.

So stop reading the Internet, go outside, and look up. Go.

Correction, April 22, 2013:This post originally misstated that the horns of the crescent moon as drawn in a Google Doodle were pointing left. They were pointing right.

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