Starship Asterisk*

BDanielMayfield wrote:The Sun illuminated both poles on the date the APOD was taken, but the view doesn’t extend clear to both poles because the satellite isn’t high enough. Since the Earth’s radius is 6,378 and the satellite (and all others in geostationary orbits) is 42,164 km from the Earth’s center the maximum latitude visible would be ~ 90 - arctan (6378/42164) = 81.4 deg.

Extra point question: How high would the satellite need to be to actually see both poles?

Nitpicker wrote:From now on, I shall happily include the month, as well as the North/South descriptors for these events.

Anthony Barreiro wrote:
The ambiguity arises because the Earth's noontime equator is moving in the opposite direction of the Sun's apparent movement in the sky. The Sun is moving south in the sky because the equator on the daytime side of the Earth is moving north of the ecliptic, and vice versa.

Eric wrote:Great pic. Has anyone been able to follow the link to the full-res image? I'm getting a "not found" when I try.
--Eric

Anthony Barreiro wrote:Lovely image of my favorite planet! ...

Psnarf wrote:You can see that rip-snorting typhoon Usagi between Taiwan and Hong Kong, as well as Pabuk following it at the edge of the disc. (It wasn't covered on the Weather Channel because they couldn't afford to send a L.L.Bean model over there to try and remain upright in 120mph winds. I imagine a drop of rain would be painful at 120mph.) On 22Sep it made landfall at Shanwei, Guangdong, China. Before weakening over Taiwan, Usagi reached category 5 @125mph. It was only 100mph when it made lanfall in China. Pabuk reached category 2, but it headed northwest and dissipated without destroying anything on land. The combined high-level moisture from those two storms crossed the northern Pacific and made folks in Seattle somewhat uncomfortable.
http://www.wrh.noaa.gov/twc/satellite/28km_wv_Pac.php (click on the image for animation)

Nitpicker wrote:Indeed they require no knowledge of the Sun. Nor do they add any.

I also prefer Northern and Southern Solstice (in June and December respectively).

Nitpicker wrote:

Anthony Barreiro wrote:Northward, Northern, Southward, and Southern are fine for those who already understand what you mean by them, but are likely to befuddle those who don't, and would be ambiguous for those who aren't sure if you're talking about the movement of the Earth's noontime equator relative to the ecliptic or the apparent motion of the Sun in Earth's sky.

Most people know the difference between North and South. For those that don't, it is easy: to find North, face South, then turn around. If that doesn't help in the slightest, not even to raise a smile, then there's not much to be done I'm afraid. Enjoy the sunshine.

I don't see the ambiguity though. Whether you think of the position of the Sun relative to the Ecliptic [Edit: sorry, relative to the Celestial Equator], or just its position in the sky for that time of year, the North/South descriptors still hold true, for everyone on the planet.

Anthony Barreiro wrote:Northward, Northern, Southward, and Southern are fine for those who already understand what you mean by them, but are likely to befuddle those who don't, and would be ambiguous for those who aren't sure if you're talking about the movement of the Earth's noontime equator relative to the ecliptic or the apparent motion of the Sun in Earth's sky.

Nitpicker wrote:

Anthony Barreiro wrote:March Equinox, June Solstice, September Equinox, December Solstice. Unambiguous and requires no knowledge of which direction the Sun appears to be moving or where the Sun appears to be standing still.

Indeed they require no knowledge of the Sun. Nor do they add any.

I also prefer Northern and Southern Solstice (in June and December respectively).

Anthony Barreiro wrote:March Equinox, June Solstice, September Equinox, December Solstice. Unambiguous and requires no knowledge of which direction the Sun appears to be moving or where the Sun appears to be standing still.

Nitpicker wrote:

neufer wrote:

Gary Loeb wrote:
I just realized my minor mistake... It IS the northern hemisphere's autumnal equinox,
as well as the southern hemisphere's vernal equinox. It's the Whole Earth's equinox.

You would think that they would have come up with better names by now.

These are the preferred terms for we upside-down people, who are easily confused:

The Northward Equinox (in March), as the Sun begins to move into the Northern Hemisphere.

The Southward Equinox (in September), as the Sun begins to move into the Southern Hemisphere.

neufer wrote:

Gary Loeb wrote:
I just realized my minor mistake... It IS the northern hemisphere's autumnal equinox,
as well as the southern hemisphere's vernal equinox. It's the Whole Earth's equinox.

You would think that they would have come up with better names by now.

geckzilla wrote:

tomatoherd wrote:

retrogalax wrote:
If i was an Extra-Terrestrial i would love to own this planet. Nice educational picture.

One group of aliens were interested once. But when they discovered the presence of septoria leaf spot, they deemed it uninhabitable.

Tomatomorphic aliens might also need to watch out for tomato hornworms. Viruses and fungi are bad enough, but nothing strikes terror into the heart of a tomato plant quite like the hornworm.

tomatoherd wrote:

retrogalax wrote:If i was an Extra-Terrestrial i would love to own this planet. Nice educational picture.

One group of aliens were interested once. But when they discovered the presence of septoria leaf spot, they deemed it uninhabitable.

retrogalax wrote:If i was an Extra-Terrestrial i would love to own this planet. Nice educational picture.

MarkBour wrote:

neufer wrote:

zbvhs wrote:
And somewhere down there, if you knew exactly where to look, you would see the satellite's shadow.

• By "shadow" do you mean the penumbra
  or the antumbra, exactly

I'm pretty certain that those numbers would quickly show that the only kind of shadow such a satellite could ever cast on the Earth would be an antumbral shadow. And its transit of the Sun would be very hard to detect, either by looking up from the Earth toward the Sun behind it (like the Kepler observatory does to stars), or by looking down on the earth for a region that is not as brightly lit. And then there's scattering in the clouds and atmosphere, which, I'm also willing to bet is plenty to completely wash out that hint of darkness, no matter how fine the detector was.

neufer wrote:

zbvhs wrote:
And somewhere down there, if you knew exactly where to look, you would see the satellite's shadow.

• By "shadow" do you mean the penumbra or the antumbra, exactly

alter-ego wrote:

neufer wrote:

BDanielMayfield wrote:
The Sun illuminated both poles on the date the APOD was taken, but the view doesn’t extend clear to both poles because the satellite isn’t high enough. Since the Earth’s radius is 6,378 and the satellite (and all others in geostationary orbits) is 42,164 km from the Earth’s center the maximum latitude visible would be ~ 90 - arctan (6378/42164) = 81.4 deg.

• How about 90 - arcsin(6378/42164) = 81.3 deg

BDanielMayfield wrote:
Extra point question: How high would the satellite need to be to actually see both poles?

The same height the geostationary satellite need to be to see either pole.

To see both poles simultaneously, the satellite must be on Earth's equatorial plane (Elektro-L does oscillates about this plane a little bit).
The simple no-atmosphere answer is infinity. The tangent planes at both poles would be parallel assuming a perfect sphere and they would only intersect at infinity. However, with the atmosphere, refraction allows for a finite distance away to see both poles.

Assuming a nominal refraction model, stars that are just visible on the horizon are actually ≈0.7° below. Using 0.7°, then the minimum distance to where both poles just become visible = 6378km/tan(0.7°) = 522,020 km, or ~1.36 x the moons orbital radius.

So it is also the case that during several days about an equinox, the sun is visible from both poles - only about 1/2° above the horizon (on a clear day of course).

dbooksta wrote:So what is that streak across the equator about 4 frames in?

neufer wrote:Polar temperature inversions near the surface might significantly enhance the refraction effect.