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I love Ned Wright's cosmology tutorial and I highly recommend it, but that statement by him is at the least very misleading. Superluminal recession speeds plainly can't be related to spacetime curvature because they don't vanish in the limit of zero curvature (zero energy density or zero $G$). The real reason that distances can be larger than $c$ times the ...


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At the heart of this question is a fundamental misunderstanding of the way gravity works in a (near) frictionless environment, such as space. Gravity pulls objects directly towards each other, which will act to slow their motion apart, or speed their motion together. However it has no effect at all on any motion at right angles to the line joining them. So ...


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The Earth's surface which is visible when you look at the planet from a certain distance is a spherical cap in terms of geometry. Here it is, in blue: $A$ is the position of the observer, $H$ is the distance from the observer to the surface of the sphere, $O$ is the center of the sphere,$r$ is the radius of the sphere, $AB$ is the distance to the true ...


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The relevant research paper is Schneider et al. (2020) "WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5: The First Extreme T-type Subdwarfs?". This gives the full designations, of which WISE 0414 and WISE 1810 are abbreviated forms. The designations contain the J2000 coordinates (J041451.67-585456.7 refers to right ascension 04h14m51.67s, ...


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This depends on eye biology, so there's no purely astronomical answer. I'll note that Venus is just about capable of casting a shadow (at greatest elongation, with ideal conditions) so let's say that a magnitude -5 object is at about the biological limit of casting a shadow. The sun has a magnitude -27, so that is 22 magnitudes brighter than Venus. 5 ...


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Yes, radio spectra have been used extensively to find the distances and locations of HI regions and molecular clouds inside the Milky Way. Observations of the 21 cm hydrogen line and/or several carbon monoxide lines (in particular, $\text{CO}(1\to0)$) enable us to make radial velocity measurements of clouds within the galaxy. From there, some geometry (see ...


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