So I’m watching some tutorials from a free online study course about astronomy. It’s run by Curtin University and the lecturer is Dr John Morgan (in case that name means something to someone).

Anyhoo, (it’s a very basic intro to astronomy) he’s talking about how the lunar eclipse told the ancient Greeks that the Earth was round, and then he said that the reason the Moon looks deep red is because of a reflection (of sorts) of all the sunsets happening around the world at the time of the lunar eclipse. This goes against my understanding… or maybe he’s just presenting it in a dumbed down version?

Divine Angel said:

So I’m watching some tutorials from a free online study course about astronomy. It’s run by Curtin University and the lecturer is Dr John Morgan (in case that name means something to someone).

Anyhoo, (it’s a very basic intro to astronomy) he’s talking about how the lunar eclipse told the ancient Greeks that the Earth was round, and then he said that the reason the Moon looks deep red is because of a reflection (of sorts) of all the sunsets happening around the world at the time of the lunar eclipse. This goes against my understanding… or maybe he’s just presenting it in a dumbed down version?

I’d imagine that the latter is the story.. The truth is easier to comprehend when connected to a reality we see every day.

i know dr john morgan and have had a couple of chats with him at astrofest.

Anyhoo, (it’s a very basic intro to astronomy) he’s talking about how the lunar eclipse told the ancient Greeks that the Earth was round, and then he said that the reason the Moon looks deep red is because of a reflection (of sorts) of all the sunsets happening around the world at the time of the lunar eclipse. This goes against my understanding… or maybe he’s just presenting it in a dumbed down version?

round because the shadow as it crosses the moon is curved and red because the reflected light is going through our atmosphere and so you get the “red sunset” type of effect due to dust and other crap in our atmosphere.

that read bad. the light that is reflected off the moon has passed through our atmosphere…

and because of the alignment the light has travelled through our atmosphere at a shallow angle just like it does at sunset and sunrise.

ChrispenEvan said:

The light that is reflected off the moon has passed through our atmosphere…

an improvement.

ChrispenEvan said:

and because of the alignment the light has travelled through our atmosphere at a shallow angle just like it does at sunset and sunrise.

getting better.

ChrispenEvan said:

that read bad. the light that is reflected off the moon has passed through our atmosphere…

That’s my understanding. The sunset thing confuzzled me.

fuck off roughbarked. if you’re just going to post drivel then go somewhere else.

the sunset thing was to give you the simile of “red sunsets” we see on earth. though on the other side of the earth it would be a sunrise.

ChrispenEvan said:

fuck off roughbarked. if you’re just going to post drivel then go somewhere else.

You don’t enjoy being appreciated for your contributions?

Methinks you may have a problem there.

what part of fuck off didn’t you understand?

ChrispenEvan said:

the sunset thing was to give you the simile of “red sunsets” we see on earth. though on the other side of the earth it would be a sunrise.

See, now that wasn’t made clear. He also lost me on Galileo’s “key insights” about motion, using a passenger on a train stopped at a station and having another train moving past as the reason we don’t feel ourselves moving through space. I think I’m overthinking it :-/

ChrispenEvan said:

what part of fuck off didn’t you understand?

The utterer.

See, now that wasn’t made clear. He also lost me on Galileo’s “key insights” about motion, using a passenger on a train stopped at a station and having another train moving past as the reason we don’t feel ourselves moving through space. I think I’m overthinking it :-/

don’t quite get that one about the trains. that is more like Einstein’s frame of reference stuff. the reason we don’t feel ourselves as moving is that there is no accelerations involved. we only feel them, changes in speed, not constant straight line motion. though i am not sure where acceleration fits in with regards circular motion.

we only feel them, changes in speed, not constant straight line motion.

we only feel them with changes in speed, not constant straight line motion.

I suspect what he was getting at was the frame of reference- we have nothing to compare to. We’re moving through space at a constant speed but have no reference for comparison (in day to day life, that is, and not looking at the stars at night). If one was born on a train and spent their whole lives on a train, they’d probably notice at some point during their life that they were moving.

ChrispenEvan said:

we only feel them, changes in speed, not constant straight line motion.

we only feel them with changes in speed, not constant straight line motion.

Without acceleration we cannot tell whether we are moving at all.

sibeen said:

ChrispenEvan said:

we only feel them, changes in speed, not constant straight line motion.

we only feel them with changes in speed, not constant straight line motion.

Without acceleration we cannot tell whether we are moving at all.

Wouldn’t the same go for deceleration? From a constant speed, I mean.

yep, though how does it work with circular motion where the constant change in direction should impose an acceleration?

ChrispenEvan said:

yep, though how does it work with circular motion where the constant change in direction should impose an acceleration?

Not a sharp enough deviation?

deceleration is acceleration, just negative. in physics they are both termed acceleration.

Not a sharp enough deviation?

wouldn’t matter. when I say “we” i mean any measurement we do not us personally as humans.

ChrispenEvan said:

yep, though how does it work with circular motion where the constant change in direction should impose an acceleration?

Centripetal force.

This is doing my head in.

Pop quiz: If Hubble Deep Space covers 1/24 millionth of the sky and can see 3000 galaxies, how many galaxies can we expect in the observable universe?

Answer: 72,000,000,000
BUT… then the answer video says that Hubble Extreme Deep Space can see an even smaller portion of sky with even more galaxies! So the “right” answer of 72,000,000,000 has to be wrong…

ChrispenEvan said:

yep, though how does it work with circular motion where the constant change in direction should impose an acceleration?

Centripetal force, and I should have expanded. You do feel a force under these conditions as there is an acceleration. This force is quite often (erroneously) referred to as a centrifugal force.

ahhh. thanks. is what i thought as well. that is why we (as humans in this case) are slightly lighter at the equator than the poles.

DA. one is deep field and t’other is extreme deep field. so they’re right with that definition.

ChrispenEvan said:

Not a sharp enough deviation?

wouldn’t matter. when I say “we” i mean any measurement we do not us personally as humans.

I think I got that. ;)

sibeen said:

ChrispenEvan said:

yep, though how does it work with circular motion where the constant change in direction should impose an acceleration?

Centripetal force, and I should have expanded. You do feel a force under these conditions as there is an acceleration. This force is quite often (erroneously) referred to as a centrifugal force.

What are you? My physics teacher?

Centripetal, pfft.

Divine Angel said:

This is doing my head in.

Pop quiz: If Hubble Deep Space covers 1/24 millionth of the sky and can see 3000 galaxies, how many galaxies can we expect in the observable universe?

Answer: 72,000,000,000
BUT… then the answer video says that Hubble Extreme Deep Space can see an even smaller portion of sky with even more galaxies! So the “right” answer of 72,000,000,000 has to be wrong…

Hubble, Hubble, toil and trouble…

ChrispenEvan said:

ahhh. thanks. is what i thought as well. that is why we (as humans in this case) are slightly lighter at the equator than the poles.

On average, the gravitational potential at sea level is the same everywhere.

If it wasn’t, the water would flow down hill until it was.

The Rev Dodgson said:

ChrispenEvan said:

ahhh. thanks. is what i thought as well. that is why we (as humans in this case) are slightly lighter at the equator than the poles.

On average, the gravitational potential at sea level is the same everywhere.

If it wasn’t, the water would flow down hill until it was.

If there existed a downslope?

roughbarked said:

The Rev Dodgson said:

ChrispenEvan said:

ahhh. thanks. is what i thought as well. that is why we (as humans in this case) are slightly lighter at the equator than the poles.

On average, the gravitational potential at sea level is the same everywhere.

If it wasn’t, the water would flow down hill until it was.

Spin of the earth makes a bulge in the ocean near the equator so sea level isn’t level.
Sun & moon gravity further confuses this.

If there existed a downslope?

Tamb said:

The Rev Dodgson said:

On average, the gravitational potential at sea level is the same everywhere.

If it wasn’t, the water would flow down hill until it was.

Spin of the earth makes a bulge in the ocean near the equator so sea level isn’t level.
Sun & moon gravity further confuses this.

Sea level is the level that the sea is at.

If it wasn’t level it would flow down hill, as it does when there is a tidal flow. That’s why I said on average.

The Rev Dodgson said:

Tamb said:

The Rev Dodgson said:

On average, the gravitational potential at sea level is the same everywhere.

If it wasn’t, the water would flow down hill until it was.

Spin of the earth makes a bulge in the ocean near the equator so sea level isn’t level.
Sun & moon gravity further confuses this.

Sea level is the level that the sea is at.

If it wasn’t level it would flow down hill, as it does when there is a tidal flow. That’s why I said on average.

This seems to differ: http://oceanmotion.org/html/background/tides-forces.htm

Tamb said:

The Rev Dodgson said:

Tamb said:

Spin of the earth makes a bulge in the ocean near the equator so sea level isn’t level.
Sun & moon gravity further confuses this.

Sea level is the level that the sea is at.

If it wasn’t level it would flow down hill, as it does when there is a tidal flow. That’s why I said on average.

This seems to differ: http://oceanmotion.org/html/background/tides-forces.htm

I don’t see anything there inconsistent with what I said.

I’m also a little fuzzy on how Cepheid variables tell us stellar distance. “Because all of the Cepheids in a Magellanic Cloud are at the same distance from us, Leavitt reasoned that the more luminous Cepheids pulsated more slowly.” How does the rate of pulsing tell us the distance?

cepheid varibles pulse at a rate that is connected to their brightness. so you measure the pulse rate and you know how bright they should be. you compare this to their actual brightness and the difference will tell you how far away they are.

ChrispenEvan said:

cepheid varibles pulse at a rate that is connected to their brightness. so you measure the pulse rate and you know how bright they should be. you compare this to their actual brightness and the difference will tell you how far away they are.

Does it have something to do with the speed at which light travels? I think this is where I’m getting stuck.

no. different pulse rates are associated with different brightnesses. so brighter cepheid pulse faster than less bright ones. you have to remember there are two types of brightness, apparent and absolute. apparent is as it appears to us on earth and absolute is how it would appear from a set distance, say 10 lightyears. so you could have a really bright star 1000ly away and it might appear the same brightness as one 10ly away. the apparent brightness is the same but the absolute would be different.

I think I get it now. I was getting caught up in light years = distance instead of luminosity (plus a bunch of maths I’ll never understand).

Now to wrap my head around Olbers’ Paradox
(These things are mentioned but not explained in the course)

absolute brightness: The apparent brightness a star would have if it were placed at a standard
distance of 10 parsecs from Earth.

apparent brightness: The brightness of an object as it naturally appears in the sky.

Cepheid variable: Star whose luminosity varies in a characteristic way, with a rapid rise in
brightness followed by a slower decline. The period of a Cepheid variable star is related to its
luminosity, so a determination of this period can be used to obtain an estimate of the star’s
distance.

Divine Angel said:

Now to wrap my head around Olbers’ Paradox
(These things are mentioned but not explained in the course)

Which, IIRC, Dr Karl talked about not so long ago.

Standard candles, they call them. Objects whose intrinsic brightness is known and can be compared with their apparent brightness, to give an approximate distance.

Olbers’ Paradox is straightforward and well explained in that Wiki entry.

Bubblecar said:

Olbers’ Paradox is straightforward and well explained in that Wiki entry.

Yes. The guy in the video made it sound much more complicated than it is.

Divine Angel said:

So I’m watching some tutorials from a free online study course about astronomy. It’s run by Curtin University and the lecturer is Dr John Morgan (in case that name means something to someone).

Anyhoo, (it’s a very basic intro to astronomy) he’s talking about how the lunar eclipse told the ancient Greeks that the Earth was round, and then he said that the reason the Moon looks deep red is because of a reflection (of sorts) of all the sunsets happening around the world at the time of the lunar eclipse. This goes against my understanding… or maybe he’s just presenting it in a dumbed down version?

He should say sunsets and sunrises, but it’s a fair (if somewhat romantic) description.