Why does the sun have to be a lot further from us than the moon? A deduction.

Remember the hands-on demo of the phase of the moon?

In both pictures: Model of the moon between my fingers on the left, and moon in the background on the right. See how the lit and dark sides of both spheres are in the same position?

Holding a sphere up in the sunlight in the direction of the moon, the sphere will show the same phase as does the moon. Of course it has to, because the sun is so far away that its rays hitting the moon and the ones hitting the sphere are pretty much parallel.

If the sun wasn’t so far away, what would we see?

Schematic of how the Earth, your little sphere you are holding up in the direction of the moon (marked X) and the Moon would be lit if the Sun was not very far away (left) and very far away from Earth and Moon. See how the phase of the moon differs from that of your little sphere when the sun is “close”?

So the only way we can explain that the lit and dark sides of the sphere and the moon are the same is that the light lighting both of them comes in parallel, which can only be the case of the sun is very very very far away compared to the distance of earth, sphere and moon.

Isn’t that a nice little thought experiment?

As frost starts melting, and the roof is getting dry, oh! The sun is up.

My office looks out directly onto the roof of our main lecture theatre, and it is fascinating how much you can observe just by looking out of a window and onto a roof.

Below is a picture of one of the first cold mornings we had this year. As the sun rose, more and more of the roof was lit and the frost melted away. Can you see where the shadow used to be just minutes ago from the shape of the still-frozen frost?



Some time later, the first corner was completely dry, while other parts of the roof were still wet, the only-recently-lit parts of the roof still had frost n them, and some parts of the roof were still frosty in the shadows.


I really enjoy making random observations that I bet most people wouldn’t even notice, but I take pictures of and write a haiku about. Good thing I have my blog :-)

The difference between secondary rainbows and double rainbows

More reflection or more rain?

Ha, aren’t you enjoying talking about optics again?

Sometimes you see two rainbows that both have red on the outside and blue on the inside. And according to my post on secondary rainbows, that should not be the case. Yet is has been observed. Why?

Screen shot 2015-06-07 at 8.09.47 PM
Rainbow and secondary rainbow, seen at Heathrow Airport. Picture by my friend F.

As you remember, secondary rainbows form outside the primary rainbows because the light is reflected twice inside the raindrop rather than only once as in the case of a primary rainbow. But that second rainbow with red on the outer rim and blue on the inner is formed differently.

Until now we’ve assumed that all the rainbows appear on the same rain front. This is not the case for the rainbow we are talking about here – it is formed on a second rain front behind the first one. So the path of light within rain drops of both rainbows on both fronts is similar, with light being only reflected once for each rainbow.

When you google double rainbows, you sometimes find pictures of two rainbows, both with red on the outer rim, nicely separated from each other. And when you see those pictures, you can be pretty sure that they’ve been photoshopped. Double rainbows of the kind we are talking about here overlap, and usually you see one full rainbow with all its colors, and then a slightly smaller rainbow with only green, blue and purple peeking out:

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If you look closely, there is a green-and-purple band on the inside of the complete rainbow. Double rainbow!

Sun dogs

Recently spotted: sun dogs, a special form of halo! Or rather sun dog (singular), since there was only one to be seen and not a second one at equal distance from the sun but on its opposite side.

Sun dog spotted somewhere between Mölln and Hamburg

These pictures are exactly as my camera took them without any filters or color enhancement or anything. Isn’t it weird that we appeared to be the only car stopping every couple of minutes to watch while everybody just continued driving?

Sun dog spotted somewhere between Mölln and Hamburg

Transit of Venus and upcoming solar eclipse on March 20th!

In 2012, I happened to be at an ESWN workshop in Madison, WI, when, during one of the breaks, one of the participants mentioned that it might be possible to get into the historic Washburn Observatory to watch Venus’ transit. Of course I had to go!

We stood in a very long queue under overcast skies for a very long time. We slowly approached the observatory, all the while watching people ahead of us go in and leave disappointed – due to the clouds all they could see when inside were live streams from other observatories. Still, there was a lot of people still in front of us. We had dinner plans and we knew that we would have to be very lucky to make it in and out of the observatory and to the restaurant on time. Half our group left in order to not be late at dinner. The rest of us stayed, still hoping.

And then we were finally inside! The observatory itself was impressive enough, but then right when we were inside, the sun broke through the clouds. All the astronomers who had been there for hours and not seen anything got super excited, as did, of course, the rest of us. Having waited all that time, knowing that we would very likely not be able to see a thing, and then coming in the moment the skies were opening up? Unbelievable. I still get excited thinking about this 3 years later.

Projection of the transit of Venus, observed at the Washburn Observatory in Madison, WI, on June 6th, 2012

The projection on the screen shown above only shows a small area of the sun, zoomed in on Venus. You can imagine the size of the projected sun by looking at the curvature on the upper left: That’s the real rim of the Sun, the rest of the circle is just due to the telescope. Watch a (very shaky) movie to get an impression of what it looked like:

So why am I telling you about this today? Because on Friday, there’ll be a solar eclipse and you should totally make sure to watch it! It won’t be a total eclipse where I’m at, but still, I’m looking forward to it! :-)

Diffraction of light

Today I’m playing with the sun.

As I mentioned in the sun dog post already, I recently went on the ferry from Kiel to Gothenburg. And I had plenty of time to watch the sun rise and set.

One thing that kept me entertained for quite a while is to squeeze the sun through the imaginary eye in the mast:


As the sun moved behind the mast (or as I moved in front of the mast, whatever), the sun seemed to get pinched in while passing.


Watch the video below to see the whole thing in action:

Also highly entertaining: Watching how the sun eats into my finger tips as I bring them together in front of the sun:

Kids. If you try this at home, please make sure you only look at the sun on the display of your camera, never look directly into the sun…

Sun dogs

More refraction of light.

Recently I found myself on the ferry from Kiel to Gothenburg, watching the sun rise.


Next to the sun, I noticed a piece of a rainbow.


Now as we all know, rainbows are supposed to only be visible when we are facing away from the sun. Clearly not the case here.


By the way, I’m talking about the rainbow-y thingy to the right of the sun, the spec close to the sun is probably something on the lens of my camera, or some other artefact of some sort.


So I read up on the rainbow-y thingy, and apparently it is called a sun dog.


There are supposed to be two of those, on either side of the sun.

IMG_9494Do you know those medieval pictures of three suns, with the outer two facing the inner one? Apparently those are supposed to be sun dogs! I never knew.


Anyway, I stood, fascinatedly watching the rainbow-y thingy.


Occasionally distracted by cool ships.

IMG_9522The higher the sun rose, the more colorful the rainbow-y thingy became. While it had been colorful for the naked eye (ok, I’m wearing glasses, but you know what I mean. No filters or polarization or anything), it started to show up on pictures, too.


In the beginning I tried finding the second sun dog on the left of the sun, but there was nothing. But the one on the right got prettier and prettier!


Eventually we arrived in the port of Gothenburg and I got distracted by container terminals and other exciting things that you will surely hear about very soon.

IMG_9551But for now I’ll leave you with this amazing view of the little islands right before you enter Gothenburg. Ready for a Scandinavian holiday? :-)


What are the ingredients of a rainbow?

Still collecting materials for our instructional short movies.

A while back I talked about how my colleague and I were experimenting with short instructional screen casts, and I shared some first attempts at movies on how rainbows form. We are still working on a story board for an improved version, but I was lucky enough to see a very pretty rainbow in a fountain the other day.

The picture below is a good demonstration of how rainbows form where there are water droplets in the air (provided there is enough sunlight, too, and we are watching from the right position) – we still see a bit of the rainbow to the right of the fountain, even though the wind direction has changed and the fountain is now blown to the left, visible because of the mist and the lower part of rainbow.

Fascinated as I was I had to film clips of this, too, which are combined in the movie below. There you see the rainbow appearing and disappearing, depending on where the fountain is moved by the wind, i.e. whether it is moved to the part of the sky where all the angles are right for us to see a rainbow, or not.

It was a magical moment – enjoy! :-)

Sun halo

A rainbow that isn’t one, technically speaking.

Browsing through the photos on my phone, I came across the one below that I took two years ago in Bergen. I remember taking the picture with Nadine on our way home, and wanting to look up what phenomenon caused this ring around the sun, but I never did – until now.

photo 3
Halo in Bergen

So, according to Wikipedia and various other internet sources, what we see in that picture is a 22° halo. In contrasts to rainbows which you see when you are facing away from the sun, this kind of halo forms as a circle around the sun. Also in contrast to rainbows, in this case sun rays aren’t refracted at raindrops, but at ice crystals. Since ice crystals typically have a hexagonal shape, this causes the radius of the halo to be on average 22°. On average, since refraction still depends on the wavelength of the refracted light – hence the halo is red-ish towards the middle and blue-ish towards the outer rim. This is also different in a rainbow, where the outer rim is red and the inner rim is blue.

Why is that? Stay tuned for the next posts, I’m still trying to figure out a good explanation. For those of you who saw my post on teaching videos a week ago, you might have noticed that I was working on something related to refraction and reflection and light already then… ;-)