The Argument with Neil deGrasse Tyson

I once had an argument with Neil deGrasse Tyson.

I should make clear at the outset that Neil deGrasse Tyson was not present for this argument. That did not prevent him from winning it.

The question was what time the Star Almanack should use to represent the night sky. My answer was midnight.

I had a good reason.

Imagine a serious astronomical observer beginning at sunset and continuing until sunrise. Perhaps the observer is attempting something like a Messier Marathon, trying to observe the 110 objects in Charles Messier’s famous catalog during a single night.

For such an observer, the night is a complete observing session.

Sunset begins it.

Sunrise ends it.

Midnight divides it.

There was an appealing symmetry to this. If the Star Almanack was going to associate a particular arrangement of the stars with a particular date, midnight seemed like the natural center of that night’s observations.

In my imaginary argument, Tyson conceded the point.

“That’s elegant,” he said.

Then he asked the troublesome question.

“Who is your observer?”

The obvious answer was the serious amateur astronomer I had imagined: someone prepared to spend an entire night moving from object to object with a telescope. But that was not the person writing the book.

When I began the Star Almanack, my practical knowledge of the night sky was modest. I could identify the Big Dipper. I could identify Orion. I could not reliably identify Polaris.

I was designing the book around the observer I imagined becoming rather than the observer I actually was.

Tyson pressed the point. How many people, he asked, go outside at sunset and remain at a telescope until sunrise?

Some do. That is real astronomy. But it is not how most people encounter the stars.

Most people go outside in the evening. They finish dinner. They walk outside. They look up.

If the purpose of the Star Almanack is to help ordinary people recognize the sky they actually see, why should its representative sky be the sky at midnight? Why not use a time when people are actually looking?

Tyson proposed 9:00 PM local apparent time.

Local apparent time is sometimes called sundial time. It follows the apparent position of the Sun at the observer’s longitude rather than the artificial boundaries of civil time zones or the seasonal adjustments of daylight saving time.

At local apparent noon, the Sun is crossing the local meridian. At 9:00 PM local apparent time, the clock of the Star Almanack therefore remains tied to the actual astronomical geometry of the observer’s location.

I still had my symmetry. Midnight divided the observing night.

Tyson had something stronger.

Tradition.

Astronomers already have a useful relationship between constellations and the evening hours. A constellation culminating—crossing the local meridian—around 9:00 PM local apparent time is naturally associated with its observing season.

So Tyson’s proposal was not merely, “People don’t want to stay up late.”

It connected three things at once.

It connected the sky to the actual position of the observer on Earth. It connected the Star Almanack to an established astronomical way of thinking about the seasonal sky. And it connected the book to the hour when its intended reader might actually step outside and use it.

My midnight convention was mathematically elegant. It was also answering the wrong question.

I had asked:

What time most naturally divides a complete night of astronomical observation?

For the observer working from sunset until sunrise, midnight remains an excellent answer.

But that was not the question the Star Almanack needed to answer.

The better question was:

What can I actually go outside and see tonight?

And the person asking that question is much more likely to be standing beneath the stars in the evening than at midnight.

Tyson won.

This was mildly irritating, especially considering that he had not actually participated in the argument. But the Star Almanack was better for it.

Its representative evening sky would not be calculated for midnight. It would be calculated for 9:00 PM local apparent time.

The symmetry I had been protecting was real.

It simply belonged to a different observer.