I am really sorry that I didn’t write much in the last time. I am preparing my first paper, and that is a little bit time and energy consuming. Nevertheless, I have to share this jewel of a paper with you: “How animals follow stars” (James J. Foster, et al. 2018). When I read the paper I was immediately on fire: animal follow stars? I could not imagine that. So I had to read it. Some people debate if our destiny is written in the stars. However, I guess nobody questions that your current location and direction are hidden in the stars above you. In history, sailors used that fact to navigate their ships in the night. Fixed stars like the Polaris can help for directions, and experienced sailors can see the latitude of their location by watching the patterns of stars. If you have a good clock with you, you can even find out the longitude (you need the clock to calculate the earth rotation which produces the same shift in star patterns as a change in longitude). So all in all, humans are able to navigate by stars. The question is if there are also animals which use the stars for their navigation. Orientation by fix stars, for example, require learning to identify individual stars by their configuration. Therefore, animals which would be able to navigate by stars, need a certain “intelligence” and “eye quality”. However, that restriction does not exclude too many species. Therefore, many different scientists analyze the behavior of many different species under the artificial sky of a planetarium or after a geographical displacement (and therefore “different” natural sky). What should I say? There is evidence that some birds can use star clues for their migration. Moreover, night-flying moths seem to orient on both the moon and the star, even though they don’t do that perfectly. Moths show a drift over time, which could be a result of lacking time-compensation for celestial rotation. Of course, there are also non-flying animals which show a talent for star navigation. For example, you can train seals to identify specific star patterns, but the question is if they use that in their natural habitat. What I found most interesting in the review of James J. Foster, et al. (2017) is the story about the ball-rolling dung beetles. Such a dung beetle does not make large journeys which need precise navigation, but nocturnal species like the Scarabaeus satyrus seem to use celestial cues to maintain their initial heading when rolling their dung ball. That prevents them from returning to their point of origin. Planetarium experiments showed that Scarabaeus satyrus use the Milky Way as the primary stellar orientation cue. Of course, the little dung beetle does not see the Milky Way like we do. An experimental study which used an artificial ‘Milky Way’ band consisting of LED lights, showed that their orientation is based on a brightness comparison. It is suggested that the beetles may identify the angle of a bright sky region or the direction of a broad-field brightness gradient. Isn’t that amazing? A small beetle using the large sky to roll their dung home safely!
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Today's paper gave me the chance to refresh my knowledge about ebb and flow (falling and rising tide). We remember: The water gets elevated by the gravitational force of the moon. This created the tidal bulge (~54cm) just underside the moon. As moon and earth both circulate around a point (barycenter) which is not in the earth center, the resulting “wobble” movement of the earth creates another tidal bulge on the opposite side (see figure). Because of the 24h rotation of the earth, any point on the earth crosses both tidal bulges over the day (~12h tidal rhythm). The tidal high varies over the month, because the gravitational force of sun also creates a water bulge (~25cm) which, depending on the moon-sun-location, either enhance the moon tidal bulge (spring tides at new moon and full moon) or counteract with the moon tidal bulge (neap tides at half moon). The standard calculation of the tidal bulge height is quite easy but it is based on some simplifications. First, the calculations are for a earth without land, which is completely covered with water. Second, only the earth and the moon contribute to the gravitational potential for points near the surface of the earth. However, the tidal bulge itself also produces a gravitational potential which should be considered. How to calculate this additional potential and how it change the end result, can be found in the paper of Travis Norsen et al. (2017). As mentioned by the authors, the idea of gravitational self-interaction of the tidal bulge is not new. The equation was already published in 1775 by d’Alamber. “But since it seems to have been— inappropriately, we think, due to the size of the effect— largely forgotten in pedagogical discussions of the tides, we thought it was worth sharing our approach with educators and students in its original format.” (Norsen et al. 2017) The gravitational self-interaction of the Earth's tidal bulge.
Norsen, T., Dreese, M., & West, C. (2017). American Journal of Physics, 85(9), 663-669. We know that the earth was attacked by asteroids and comets in its history. We just have to remember the sad ending of the dinosaurs. However, even if there were many asteroids and comets falling on earth, it is hard to study them. Erosion and geological changes like volcanos and water makes it hard to identify the craters and to study its age. Much better are the circumstances on the moon. There is no water and no volcanos. Therefore, the craters stay “untouched”. And when there would be a time in which many asteroids and comets attacked the moon, it just seems logical to assume that in the same time there were also asteroids and comets falling on earth as they are close neighbours. Therefore, knowing the history of the crater impacts of the moon also reveals some informations about the history of our earth. Interestingly, lunar probes collected by Apollo astronauts showed that around 3.9 billion years ago the crust of the moon was so hot that all rocks in it metamorphosed. This was the birth of the lunar cataclysm hypothesis: the heating up of the crust may be created by a large number of asteroid and/or cometary collisions in a brief pulse of time. However, did the lunar cataclysm really happen? The Apollo astronauts just collected probes in a small area so just because these probes are from the same age, it has not to be valid for the whole moon surface. However, in December 2000, Cohen et al. published a study which results support the hypothesis. They studied the ages of 4 lunar meteorites which were found on earth. The lunar meteorites are like random samples from the moon surface and indeed the scientists found in them nothing which was older than 3.9 billion years. So that supports the hypothesis that at this time point many collision events happened on the moon. And maybe this also affected our earth. Remembering that the earliest isotopic evidence Scientists are now wondering if this cataclysmic bombardment may have affected life on Earth or been involved in life's origins. The earliest isotopic evidence of life is ~3.8 billion years old. Can this be an accident? (please read here more about it: http://www.lpi.usra.edu/exploration/science/lunarCat/ ) "Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages."
B. A. Cohen, T. D. Swindle, and D. A. Kring. Science 290.5497 (2000): 1754-1756. |
IdeaI love to increase my general science knowledge by reading papers from different fields of science. Here I share some of them. Archiv
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