I was taking photos of the starry sky with a Canon 1DX DSLR and a 400MM lens. I was taking long exposures (2 seconds each) to allow the faint stars to show up. I noticed a dark shadow on one of the photos on the screen, and took more photos. The next photo showed the same object, but it had moved, and appeared to be moving in a curved path. I kept shooting, but the object did not appear in any more photos.I was using a professional tripod, and the camera was not moved between any of the shots. The two shots that contained the dark spot were taken within a few seconds of each other. I have no idea what this could be, but I can tell you a few things that it is NOT. It is NOT something on the lens, because it would be so blurred as to not be visible. The focus was set at infinity, and anything closer than a couple hundred feet would be so blurry that it would not be noticeable at all. It is not anything on the lens, because with this lens, and set at infinity, I can place one finger on the lens and you will not see ANY shadow. Virtually the entire image gets a bit darker, as the finger is SO out of focus. If the spot did not move, it could be claimed that it was dust on the sensor itself, but dust on the sensor does not show up for two photos, and move on it's own while the camera is on a tripod not moving, and then disappear. I've had dust on the sensor plenty of times. Once it shows up, it stays there until you clean it off, and it NEVER moves.The object appears to be either standing still or moving VERY slowly across the field of view, so this eliminates any kind of flying animal, at any height. Again, for the shadow to be in as much focus as it is in would require the object to be hundreds of feet away, and as such, much larger that any bird or bat. I could see a scenario where a helium balloon or two tied together might cause in image like this, but the apparent motion is not in a straight line, and with the almost non-existent wind, I can't imagine a balloon moving in what appears to be a curved flight path. if it was gusty, I could see a balloon moving like this, but the slight breeze was only about 1-2 MPH, and at times it was dead calm.There was absolutely nothing visible to my naked eye, but given that this was a long exposure, I would not expect to be able to see the object, even if I were to be using binoculars. keep in mind that this was a 2 second long exposure, so there would be some motion blur on the object even if it was in perfect focus, if it moved at all during the exposure. I heard nothing, and never saw anything unusual with my eyes. I took many more photos, but nothing anomalous showed up before or after these two. I am attaching the unedited raw files. These are Canon's raw format, so you might need to convert them to a DNG or JPG, but I will leave that to you so that you know I have not edited these at all.