Hacker News Reader

Comment by jcims | Hacker News Reader

(Edit: To get an idea what this would look like if you were to watch this star moving across the sky like a comet)

The sun's apparent diameter in the sky is roughly a half a degree. Over the course of an hour, it will move approximately 15 degrees across the sky, or rougly 30 apparent diameters. The actual diameter of the sun is ~860,000 miles, making the 'apparent velocity' of the sun across the sky nearly 26 million miles per hour or ~6 times faster than this beast.

I mention that to apply wet socks to any notion of a bullet star flinging across the sky like tracers at the Knob Creek night shoot. It's fast yet, but at that scale, doesn't look it.

(for the uninitiated - https://www.youtube.com/watch?v=h11uEvGc7u8 )

View on HN

I think I had a better intuition for how fast this star would appear to move across our sky from the headline alone (which is just as well because I can't see the article) than how fast "tracers at the Knob Creek night shoot" might do.

Agree. Is Knob Creek a place from history, a place from a video game, a place from a movie, a place entirely made up?

I thought it was a whiskey

The 15 degrees comes from the Earth's rotation, right? Not sure what you're trying to compare here. If I spin in a circle at 1 second per revolution, I guess I can make the sun travel about 2.2 billion miles per hour using your numbers.

> any notion of a bullet star flinging across the sky like tracers at the Knob Creek night shoot. It's fast yet, but at that scale, doesn't look it.

That would appear to be what the poster is trying to compare, there.

Bingo

Yeah just trying to connect how this would look visually if this star were to fly across the night sky like a comet.

A web search says your approach (which I admit I don't follow) is off by a factor of 40 -- the Sun moves approximately 500k mph around the galactic center. A speed 8x slower than the star mentioned in the article.

The GP is talking about the sun’s apparent speed in Earth’s sky, which is dominated by Earth’s rotation. Another way of stating it:

The sun is 1 AU distant and appears to revolve around Earth once a day from our perspective so it has apparent speed of 6.28 AU / day or 2.4 million mph.

Their point is that unless this star was very close to us it wouldn’t appear to move quickly across the sky, because the sun has similar apparent velocity and looks stationary to our eyes.

yep!

"Quick" being relative to your expectations, of course.

If OP is right, and it moves six times faster than the sun, then it would go from one side of the sky to the other in just three nights, right? That's extremely fast for any star gazer. Faster than any planet we can see. Someone who was familiar with the night sky and who could see this star would notice its speed quickly.

Reply on news.ycombinator.com