APOD: A Five Planet Dawn (2016 Jan 30)

[APOD Robot]

A Five Planet Dawn

Explanation: As January closes and in the coming days of February, early morning risers can spot the five naked-eye planets before dawn. Though some might claim to see six planets, in this seaside panoramic view all five celestial wanderers were found above the horizon along with a bright waning gibbous Moon on January 27. Nearly aligned along the plane of the ecliptic, but not along a line with the Sun, the five planets are spread well over 100 degrees across the sky. Just arriving on the predawn scene, fleeting Mercury stands above the southeastern horizon in the golden light of the approaching sunrise.

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[Boomer12k]

What a sight that must have been in real life...awesome.

:---[===] *

[heehaw]

I vividly remember, decades ago, seeing Mercury while at a meeting at Los Alamos. Not the only time I've seen Mercury, but by far the best: clear and bright and high in the western sky after sundown with no trace of clouds anywhere in the sky. Mercurial Mercury had been caught! It is a small planet but it is a VERY dense one. A day on Mercury is two years long. They insist that ice is detected on Mercury. I find Mercury very mysterious!

[Startreader]

A Five Planet Dawn

Seven.

Unless the IAU has changed its tiny mind again and eliminated the Earth.


Not that I believe in Mercury. I've never seen her in real life, only images. Like M31, I suspect she's a figment of the telescope.

[Chris Peterson]

What a sight that must have been in real life...awesome.

Well, you can see it for yourself just going out before dawn (with the possible exception of Mercury, depending on your horizon). I've seen it a few times already, and it's very nice. Still, I wouldn't qualify a string of planets (only two bright) stretching halfway across the sky as quite reaching my threshold for "awesome".

[Chris Peterson]

A Five Planet Dawn

Seven.

Unless the IAU has changed its tiny mind again and eliminated the Earth.

Five planets, six if you include Earth. There's no reasonable way to say seven unless you are going back to the ancient tradition of including the Moon (which the APOD editors were not).

Of course, you could make a case for Pluto, which is also in this image, contributing a few photons just below Mercury... although they are most certainly lost in the noise.

[Sally]

I'm curious about what is meant by this line in the text: " Nearly aligned along the plane of the ecliptic, but not along a line with the Sun,"

I know what the plane of the ecliptic is but I don't understand what is meant by "not along a line with the sun". Can anyone elaborate on this?

Thanks!

[Chris Peterson]

I'm curious about what is meant by this line in the text: " Nearly aligned along the plane of the ecliptic, but not along a line with the Sun,"

I know what the plane of the ecliptic is but I don't understand what is meant by "not along a line with the sun". Can anyone elaborate on this?

That's a good question, considering that the Sun defines the ecliptic, and in fact, all of these planets (and the Moon) come very close to defining a line that passes directly through the Sun.

All I can guess is that the comment is somehow related to the fact that in this projection we see the line of the ecliptic as curved. But if you follow the curve from the Moon to Mercury, and extend it in your mind, it will pass right through the Sun just below the horizon.

[FrankAllen]

I think it would be really neat to take a series of time-lapse photographs each morning, and then string them altogether to make a movie so that you could actually see the planets moving along their orbital paths. Anybody up to doing that? I don't have the equipment to do, but surely somebody out there does.

[Coil_Smoke]

I hope to view Mercury, someday. When will, or did, Mercury reach greatest elongation this time around ?

[Chris Peterson]

I hope to view Mercury, someday. When will, or did, Mercury reach greatest elongation this time around ?

On February 8. But it's essentially at a standstill now.

If you want to see it the easy way, wait until May 9. Except for a narrow band of the west Pacific (which includes Australia), you can observe Mercury transit the Sun. All you need is a small telescope or a pair of binoculars fitted with white light filters. It should even be visible to the naked (filtered) eye.

[trekman]

I dont know enough about astronomy or space to know the answer to this. Ive always been amazed at the infinitesimal number of things in the heavens. How is it that we can look at 5 specks in the sky and know exactly what planet or what star they are? Thanks.

[Chris Peterson]

I dont know enough about astronomy or space to know the answer to this. :( Ive always been amazed at the infinitesimal number of things in the heavens. How is it that we can look at 5 specks in the sky and know exactly what planet or what star they are? Thanks. :)

We know what star is which because they are (to a large degree) static. They maintain the same position with respect to other stars, forming patterns that we can recognize. More technically, they maintain a position against the sky coordinate system that is fixed, just like a town or lake has a fixed latitude and longitude.

The planets are identified by position, as well, although how they move and how they appear (especially in a telescope) can also be considered.

[Cousin Ricky]

Though some might claim to see six planets, ...

Who wouldn't claim that? B.o.B?

[ta152h0]

Can APOD get an artist to " paint in " the rest of the planets on this image ? A annotaded spot is fine by me

[Nitpicker]

Can APOD get an artist to " paint in " the rest of the planets on this image ? A annotaded spot is fine by me

Uranus and Neptune had already set in the early evening and didn't rise again until well after the Sun. Planet Pluto is near Mercury, though too dim to be visible, as has already been mentioned.

[Startreader]

A Five Planet Dawn

Seven.

Unless the IAU has changed its tiny mind again and eliminated the Earth.

Five planets, six if you include Earth. There's no reasonable way to say seven unless you are going back to the ancient tradition of including the Moon (which the APOD editors were not).

Of course, you could make a case for Pluto, which is also in this image, contributing a few photons just below Mercury... although they are most certainly lost in the noise.

If we take the IAU to be the final arbiters of planethood, and there is no reason we should, then the Earth and the Moon are either both planets or both something else. The Moon sort of doesn't really orbit the Earth, it co-orbits Sol with the Earth and the Moon is larger, by far, in relation to its co-orbiting partner than any satellite of any planet.
Yes, some asteroids (dwarf planets, minor planets) have partners that are closer together in relative sizes as does Pluto but the IAU insist that none of those are planets.
Luna and Earth have, together, mostly cleared their common orbit, certainly more than Jupiter has and they are both large enough to be nearly rounded by gravity. What more could we wish of a planet?

If Luna isn't a planet, neither is Earth.

Anyway, Luna probably used to be a totally independent planet before the two orbits got slowly synchronised.


Full disclosure: I think the IAU are wrong, Pluto is a planet. Ceres and Vesta are also planets. Eris and the others are not. Why? Because that makes me happy.
I also consider Luna, the Moon, to be at least so much a planet as the Earth.

[Chris Peterson]

If we take the IAU to be the final arbiters of planethood, and there is no reason we should...

Well, the IAU is the final arbiter of the technical definition of "planet" as used by astronomers in professional communications. There are, of course, many other equally valid definitions used outside of that context.

... then the Earth and the Moon are either both planets or both something else.

Most astronomers would disagree, given that the barycenter of the Earth-Moon system lies inside the Earth, making the Moon reasonably described as a satellite of the Earth, regardless of their solar orbits.

[Cousin Ricky]

I was about to compose a rebuttal, but then you wrote this:

Why? Because that makes me happy.

Sorry, science doesn't operate that way.

All other rebuttals are rendered superfluous.

[ta152h0]

Rebuttals are futile with Mr Peterson

[ta152h0]

and so is resistance

[Startreader]

Most astronomers would disagree, given that the barycenter of the Earth-Moon system lies inside the Earth, making the Moon reasonably described as a satellite of the Earth, regardless of their solar orbits.

So, if the Earth were slimmer and denser, so that the barycentre was outside the planet it would be a double world? Or when Luna's orbit is tidally enlarged to a size where the barycentre is well outside the planet they will become a double world?
A little arbitrary, yes?
No other planet has a satellite so large in comparison to itself. Luna's orbit is closer to the ecliptic than to Earth's equator, unlike other large, regular satellites and Luna never goes "backwards" relative to its orbit around the Sun.
Luna's a planet.
It was a planet before it co-orbited with Earth, it will be a planet when its orbit gets wider and it's a planet today.

Pluto-Charon is a double-world, too, and Vesta and Ceres are planets. Not that it matters. I am never going to change the minds of the IAU committee members or the professional astronomers.

[Startreader]

I was about to compose a rebuttal, but then you wrote this:

Why? Because that makes me happy.

Sorry, science doesn't operate that way.

I know. I used to do Science. But defining a planet isn't Science, it's politics. The current "official" IAU definition was dreamed up and voted on by a half-a-dozen members of one organisation and it allegedly made none of them happy. It was a typical political compromise grudgingly accepted by a committee trying to get the day's business done before the bars closed.
My definition makes at least one person happy.
Therefore the greater benefit to Humanity in general would be made by adopting my definition.

All other rebuttals are rendered superfluous.

So, had it not made me happy you would have given me a lengthy, reasoned rebuttal? That's a bit sad.
I thought the whole point of Science was to better the Human condition? Otherwise why bother? My definition betters the Human condition more than does the IAU's by the amount of the happiness of at least one person.
It's depressing that this counts for naught.

It doesn't matter, anyway. I surrender. You are right, I am completely and utterly wrong and I'm gone.
Bye.

[Chris Peterson]

But defining a planet isn't Science, it's politics.

Actually, it's a bit of both.

[geckzilla]

I think a lot of people agree that the IAU's definition of a planet could use some more refinement. However, "because it makes me happy" is not a very good way of going about it. Under the current definition, exoplanets aren't even planets, which is somewhat laughable.

Take for example the case of a binary star system: A system like our own, except in place of Jupiter is a small star. Now, around this small star is another orbiting body, a rocky orb with a thick atmosphere. Is it a planet? Of course, this is just a setup for me to tell you that Titan ought to be a planet, because I've just described Titan except that I've moved it to Jupiter and turned Jupiter into a star.

TITAN FOR PLANETHOOD