Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

From a million miles away, NASA camera shows moon crossing face of Earth

nasa.gov

Post reply on HN
Print view View original

51–60 of 159 posts

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#51

Post by TeMPOraL »

mnw21cam wrote: post #21
bumbledraven wrote: post #16

Earlier quoted context omitted.

The satellite is 1 million miles away from Earth at the L1 Lagrange point. What is it about optics that allows us to compare a photo of the stars taken at such a distance with one taken from the surface of the earth? [Edit: "compare", in the sense of understanding whether or not the stars will or won't show up in each case. I get that the laws of optics are the same throughout the universe.] > The dynamic range diffe…

Yes, our eyes just do have a fantastic dynamic range. Having said that, if you were sitting on the satellite, you would see even more stars if you blinkered out the brightness of the sun, moon, and earth.

Don't cameras have much better dynamic range, but we just limit it in software by reducing everything into 8-12bit/color images? After all, the hardware is just a bunch of sensors counting photons.

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#52

Post by bhaak »

Tobold wrote: post #43

Someone tweeted this yesterday and some of the replies were really disheartening. Several people were calling it fake, with varying explanations of either "The earth is flat" or "Everybody knows the whole space program is fake". I knew people out there actually believe this, but damn...

I have to say that the images look like badly photoshopped fakes to me as well. If they weren't on a NASA page, I wouldn't believe them to be real.

Sometimes, reality is quite different from what we expect it to be.

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#53

Post by Sakes »

gurtwo wrote: post #32

Why don't we see the Moon's shadow projected over the Earth in the pic? I guess it has to do with the relative distances of the 2 bodies to the Sun and the Moon's penumbra or something. Can someone make a digital image simulation of this to verify?

When the moon casts a shadow on the earth we (on the surface) experience it as a solar eclipse.

For these images, the moon is not directly between the sun and the earth, so it cannot block sunlight from hitting the earth. So, no shadow.

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#54

Post by jeffwass »

gojomo wrote: post #36
gurtwo wrote: post #32

Why don't we see the Moon's shadow projected over the Earth in the pic? I guess it has to do with the relative distances of the 2 bodies to the Sun and the Moon's penumbra or something. Can someone make a digital image simulation of this to verify?

I thought the dark edge at the top-left of the moon was a shadow, but there was no solar eclipse in the time-frame reported for these pictures. So, even though a perfect alignment of DSCOVR (at L1), the moon, and Earth would mean a shadow and thus eclipse, the moon must be a little off that alignment on this pass, throwing its shadow into dark space, even though it looks very nearly aligned in the photos.

That dark edge is an artefact of the delay in capturing the three monochrome pictures through the different RGB filters.

The article says : "Combining three images taken about 30 seconds apart as the moon moves produces a slight but noticeable camera artifact on the right side of the moon. Because the moon has moved in relation to the Earth between the time the first (red) and last (green) exposures were made, a thin green offset appears on the right side of the moon when the three exposures are combined. This natural lunar movement also produces a slight red and blue offset on the left side of the moon in these unaltered images."

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#56

Post by jsingleton »

geofffox wrote: post #9

Maybe as interesting is where this satellite is: The Lagrange L1 point. The 5 Lagrange points in essence allow orbiting bodies to break Kepler's laws. A satellite a million miles closer to the Sun shouldn't have the same orbital period as the Earth... but it does.

L1 in particular is perfect for measuring the solar wind and Coronal Mass Ejections as an early warning system [0]. DSCOVR (the probe that took this photo) is the successor to ACE which is used to predict Auroras amongst other things [1]. This data is released by NOAA as a forecast [2]. I built an Aurora simulator with a similar API [3], but I think that mainly uses magnetometers.

[0] https://en.wikipedia.org/wiki/Lagrangian_point#Spacecraft_at...

[1] http://www.nesdis.noaa.gov/DSCOVR

[2] http://www.swpc.noaa.gov/products/aurora-30-minute-forecast

[3] https://unop.uk/dev/led-strip-aurora-alerts

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#57

Post by jeffwass »

rogeryu wrote: post #13
aspirin wrote: post #2

Is this the real reflectance of the moon? Seems so dark. Usually we see the moon at night surrounded by night sky, which probably skews our perception of the real color of the surface material.

Probably because that's the Dark Side of the Moon? https://en.wikipedia.org/wiki/The_Dark_Side_of_the_Moon

"There's no dark side of the moon, really. Matter o' fact, it's all dark."

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#58

Post by acqq »

TeMPOraL wrote: post #51
mnw21cam wrote: post #21

Earlier quoted context omitted.

Yes, our eyes just do have a fantastic dynamic range. Having said that, if you were sitting on the satellite, you would see even more stars if you blinkered out the brightness of the sun, moon, and earth.

Don't cameras have much better dynamic range, but we just limit it in software by reducing everything into 8-12bit/color images? After all, the hardware is just a bunch of sensors counting photons.

Go here and see for yourself, for example: http://www.dxomark.com/Cameras/Nikon/D810---Measurements

ISO50, 14ev, ISO 12800 8ev. Then read here:

http://photo.stackexchange.com/questions/21579/how-does-the-...

We as humans can probably obtain 24 stops when our body uses all the tricks it has, not only optics of our "hardware".

Re: From a million miles away, NASA camera shows moon crossing face of Earth

#60

Post by acqq »

TeMPOraL wrote: post #50
acqq wrote: post #15

Earlier quoted context omitted.

Note that the Moon is fully lit by the Sun (it's a "full Moon" from the perspective of the satellite). And it's gray on the picture, as the Earth is significantly brighter. Now, you can check it yourself, take the best camera you can have, go one night when the Moon is full somewhere outside of the city where you see the stars and then try to make a photo where on the photo both the stars and the Moon surface are vis…

Isn't this just because of the processing? I imagine if you took additional logarithm of the values returned by the sensor, you could have both starts and the bright moon visible at the same time.

No, our bodies can simply do more, see:

http://photo.stackexchange.com/questions/21579/how-does-the-...

We could use additional "tricks" with cameras too (e.g. physically obscuring the moon and earth for one frame to shoot the stars then mixing it with the next photo) and with the additional post-processing (which would certainly include reducing the dynamic range to be able to show the photo on the screen) get something that some skeptics would think is "normal" but it's not what was being done on the photos presented, as nobody designs that much for features that actually don't bring anything: It would be easier to shoot just the sky in the same direction (the stars would look effectively the same, they are that far -- the satellite is just one hundredth of one AU away from the Earth and Alpha Centauri is some 271 thousands AU away) and photoshop it.

Post reply on HN