# Terry Tao’s MoMath lecture part 3: The speed of light and parallax

[sorry if this doesn’t read too well.  woke up sick today and am getting sicker.  boo 😦 Didn’t have the energy for too much editing. ]

In the last few weeks I’ve been writing about Terry Tao’s incredible public lecture delivered at the  Museum of Math over the summer and how that lecture provides many great examples you can use to talk about math with kids.  The first two posts are here:

Part 2 of using Terry Tao’s MoMath lecture to talk about math with kids – Clocks and Mars

Part 1 of using Terry Tao’s MoMath lecture to talk about math with kids – the Moon and the Earth

for ease, the direct link to the Terry Tao lecture  is here:

Today we were talking about the piece of the talk starting around 1:04:30 – how physicists obtained the first estimate for the speed of light and also  how astronomers measured the distance to nearby stars.

We began by watching Terry Tao’s presentation and then discussing the boy’s reaction to the video.  They seemed to have a reasonably good understanding of how the measurement of the speed of light was done.  Their ability to understand the talk is why I think Tao’s lecture is so great for kids to see – his explanations are incredibly easy to follow.  We had to clarify a few points, but after those clarifications we were able to repeat the calculation.

Also, working through the calculation is a nice exercise in place value and division for kids.

Following that conversation we moved on to the discussion of how astronomers measured the distance to the nearest stars.  The portion of Tao’s lecture that discusses parallax is amazing, but one really interesting thing that his pictures don’t really illustrate are all of the distances involved.  I draw a similar picture to the one Tao used in the talk and then mention what the proper proportions would be at the end of the video.

Also, at the beginning of this video my younger son was confused by the distances I had written down for the radius of the Earth and the radius of the Sun.  I’m not sure exactly what was bothering him, but since a critical point for understanding parallax is understanding distances, we spent a few minutes at the beginning of the video making sure he understood those distances properly.

Finally, we went out to the back yard to demonstrate the relative distances involved in the measurement of the distance to Alpha Centauri.  We used a small balloon with radius about 2.5 inches for the sun and a grain of salt for the Earth.  At this scale the radius of the Earth’s orbit around the sun is about 50 ft.    Also at this scale, if we were standing in New York City Alpha Centauri would be in Los Angeles!   Sorry for all the coughing in this one – I’m a little sick today 😦

So, one more neat project for kids coming from Terry Tao’s lecture.  It is a little hard to go into the details of how the angles were measured since you need trigonometry for that, but the geometry is easy enough to understand.   Attempting to “draw” the picture to scale in our back yard was really fun, too.  The calculation of the speed of light really just requires a little arithmetic and is a nice example to show to help build up number sense.

Definitely a fun morning!