# Why does DSCOVR's camera EPIC see at least 13 sunrises and sunsets per day?

In this NASA Goddard YouTube video titled "One Year on Earth – Seen From 1 Million Miles", I've gotten stuck on the line

In this view, EPIC sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day.

edit: Listening carefully, it's "...see the sunrise... and the sunset..." and not "...see the Sun rise... and the Sun set...", so my transcription is not correct. I'll leave it, along with this note, because the distinction is important. I only caught it after reading the nice, concise answer by @Leorex through carefully.

DSCOVR is in a Lissajous orbit about the Sun-Earth L1 Lagrange point between the Earth and the Sun, and the EPIC camera points towards the earth which remains nearly 100% sunlit.

A given point on earth sees about one sunrise and one sunset per day, and the Lissajous orbit gently rocks back and forth slightly with roughly a six month period.

I don't understand what it is that is happening at least thirteen times a day. Can someone help me understand this?

above: Screen shot during the quoted line. Start playing at 00:44 to hear it.

• Hmm, perhaps it has something to do with the photo rate of the camera? Maybe it only takes 13 pictures a day. – Phiteros Jul 24 '16 at 0:17
• @At least in the beginning the photo rate was pretty uneven, maybe it stable now, and that seems to sound very familiar. So you mean that in every image, there is almost always a leading and trailing edge of the earth, and so every image contains a spot on the earth which is experiencing sunset and a sunrise? That wouldn't be exactly true, since DSCOVR is usually leading or trailing a bit, so one of the two is actually out of view most of the time. – uhoh Jul 24 '16 at 0:22
• They may not be trying to be very accurate here. Most people wouldn't realize that DSCOVR captures a few miles short of the actual place the sun is rising or setting. – Phiteros Jul 24 '16 at 0:25
• @ more than a few miles - like +/- 10 degrees in the plane of the ecliptic as seen from earth from this nice DSCOVR presentation. If you pause the video, then click between say 02:04 and 02:34 you can see the "chunk of missing earth". But OK I know what you mean. Do you think you can post this as an answer? – uhoh Jul 24 '16 at 0:42
• I watched this video yesterday and have been wondering about the exact same question. – Stack0verflow Jan 31 at 20:26

In this view, EPIC sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day.

This is incorrect. This statement is based on the planned activity of sending home at least 12 images a day. Communication link restrictions and the higher priority of the scientific instruments on DSCOVR over EPIC means that this does not always happen. Saying that EPIC takes 12 images per day does not sound very exciting. Saying that EPIC "sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day" somehow does seem exciting. It is however deceiving. Because DSCOVR is in a Lissajous orbit about the Sun-Earth L1 point, almost all of the portion of the Earth visible to DSCOVR (and hence to EPIC) is fully sunlit.

Technically, this answer is quite incorrect. A single EPIC image occasionally contains a tiny, tiny part of the Earth that is in sunrise or a tiny, tiny part of the Earth that is in sunset, but only very rarely both. EPIC does not see "the Sun rise in the west and the Sun set in the east, at least 13 times a day." Because of atmospheric effects, sunrise occurs before any part of the Sun is geometrically above the horizon, and sunset occurs after every part of the Sun is geometrically below the horizon. To simultaneously see sunrise and sunset would require seeing more than half the globe. Simultaneous sunrise and sunset cannot be observed simultaneously from DSCOVR's perspective because it sees slightly less than half the globe.

Editorial comment: It would have been much better to simply say that "In this view, EPIC sees almost half the globe, almost all of which is fully sunlit. Because of the Earth's rotation, and because EPIC takes at least 12 images per day, it sees almost every part of the sunlit Earth, every day."

A different satellite (which does not exist) could provide a lot more than 12 fully-sunit images per day, but that satellite does not exist. Twelve pictures a day might not be that much, but it is a lot better than none.

• @DavidHammen some times it takes me a while, but I fully understand your answer now. You've brought up several issues here, all of which I now understand. Yikes, this really was an unfortunate choice of words in the narration, and I have to admit - they are wrong no matter how obliquely one listens to them. – uhoh Feb 7 '17 at 2:45
• I have trouble with some of the answers above. I now understand that all they meant, was that each image has a sunrise and sunset on it. However, you don't need to see the whole globe for the sunset to be on it: when we, on Earth, see the sunset, we are looking at the sky very far away from us that is above the Earth a small partway around the globe, rather than being at a point directly above us. So you don't need 100% coverage in the photo for it to be there. And at 1 million miles away, there isn't much missing from that angle! – Timmi Jun 27 '17 at 5:49

In your embedded YouTube video, at 1m 16s EPIC's Lead Scientist Jay Herman says, "Epic takes at least one set of images about every two hours."

If Epic takes a set of images every 1h 50m 46.153s, it will take 13 sets of images in 24 hours, each showing a sunrise and a sunset.

• Since DSCOVR moves about +/-275,000 km projected in-plane, would you say that at least half the time one of those is on the far side of the earth and not visible? Even more so because DSCOVR is not at infinity? If it were at L1 instead of in a big orbit about L1, wouldn't pretty much all of the terminator always be invisible? – uhoh Jul 24 '16 at 2:14
• Nah, this is the correct answer despite the technicalities. I think my original error in the transcription - thinking it was "...seeing the Sun rise..." when it was actually "...seeing the sunrise..." that threw me off. A two sentence answer containing a full explanation plus the source of my confusion should get extra credit! – uhoh Jul 24 '16 at 2:41
• I was a bit slow in gather extra information, initially I thought you were asking more about "thirteen" instead of how could it possibly see east/west terminators at all times throughout the year. I suspect they mean "beginning/ending stage of a period" rather than specifically "the ascent/descent of the sun above/below the horizon". (But I was checking into the polar regions to see if there was a technicality.) – Leorex Jul 24 '16 at 3:01
• I originally thought the sun rose thirteen times a day, but it was BC (before coffee) for me here, or at least before enough of it. It's a beautiful satellite, video, and geometry problem all in one. Thanks! – uhoh Jul 24 '16 at 3:06
• I've un-accepted this answer although it did in fact solve my problem by helping me understand my mistake. I have to agree with the other answers that it does not show the sun set, but neither does it show a sunset (ditto for '-rise'). At best it shows one or the other terminator, and if the sky were clear, and there were populated land or ships or aircraft present, people at those locations might be able to observe the sun rising or setting. – uhoh Feb 7 '17 at 2:39

I've accepted the great answer by @Leorex. I went back and found that the video has closed captions (the little [CC] thing) and indeed my original transcription was wrong.

EDIT: and then I looked at the whole thing again six months later, after this answer and decided that no, the problem is more than just confusing "sunset" for "sun set". @DavidHammon's logic is inescapable, as usual.

• Well I think DSCOVR and it's camera EPIC are just great, and I never cease to be amazed and proud of NASA. Outreach is critical and it has to hit all the notes on the scale. I originally didn't understand one sentence so I wrote this question. It turns our it was a small mis-transcription on my part and with the help of the SX SE community I got it figured out. Thanks everyone! – uhoh Jul 27 '16 at 12:51

This may be less confusing: The camera shot includes either the sunrise terminator in the west or the sunset terminator in the east, at least 13 times per Earth solar day.

• Yep, thanks for that. It's1) 'terminator' not sunrise/susnet or 'sun rise/set', and 2) 'or' not 'and'. Though as pointed out by @DavidHammon due to the geometry of DISCOVR being much closer to the Earth than the sun, there is actually a significant chunk of the Lissajou orbit near the Earth-Sun line where neither is visible. See this answer or just this drawing: i.stack.imgur.com/5fyLc.jpg – uhoh Feb 7 '17 at 2:57