Jupiter and Saturn, the two largest planets in the Solar System, will conjoin very closely near Christmas this month, putting on an astral show that hasn’t been seen on Earth for roughly a millennium.

The two planets on Dec. 21 will appear closer together in the sky than at any time in the last 800 years.

Alignments of the two planets are relatively rare as far as astronomical events go, but the last time Saturn and Jupiter were this close was in March of 1226, Rice University astronomer Patrick Hartigan told the Daily Mail.

Legendary astronomer Johannes Kepler speculated in the 17th century that the “Star of Bethlehem” that reportedly guided the Three Wise Men to the birthplace of Jesus Christ may have been a similar conjunction of Jupiter and Saturn.

Arecibo Telescope's illustrious scientific career is over.

The National Science Foundation (NSF) will decommission Arecibo Observatory's massive radio dish after damage has made the facility too dangerous to repair, the agency announced today (Nov. 19).

The announcement came as scientists awaited a verdict about the fate of the iconic observatory after damage to the complex cabling supporting a 900-ton science platform suspended over the dish. In August, a cable slipped out of its socket, but engineers evaluating the situation deemed it stable; earlier this month, a second cable unexpectedly snapped, leaving Arecibo's fate much more perilous. After considering three separate engineering reports, the NSF, which owns the property, has decided the facility is unstable enough that there is no way to repair the damage that does not put personnel at undue risk.

Period: Dec 4–17
See table of times
Peak: Dec 13–14 in Texarkana
Up to 120 meteors per hour
Parent: 3200 Phaethon
Where: Both Hemispheres

Sometimes, it behooves us to consider a larger perspective.

Here is one such perspective. On September 5, 1977, NASA launched Voyager 1, a deep space probe designed to study the outer Solar System. This device performed far better than expected. As I write this, the probe is over 13.6 billion miles from Earth, the farthest of any devices ever launched by man…and is still responding to commands.

By 1990, Voyager 1 had completed its primary mission and was heading out of the Solar System. However, it was to get one final and most significant mission. Astronomer Carl Sagan prevailed upon NASA to have the spacecraft take a “family portrait” of the planets in the solar system, including Earth. The pictures were taken on Valentine’s Day, 1990 from a distance of 6 billion miles from home. The one showing Earth is the subject of a book by Sagan. No thinking person can see that picture, read his words, and then logically conclude that this all “just happened” on its own.

We succeeded in taking that picture [from deep space], and, if you look at it, you see a dot. That’s here. That’s home. That’s us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives.

The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every inventor and explorer, every teacher of morals, every corrupt politician, every superstar, every supreme leader, every saint and sinner in the history of our species, lived there on a mote of dust, suspended in a sunbeam.

Carl Sagan, 1994

On 28 August 1859, a series of sunspots began to form on the surface of…

Dozens of times over the last decade NASA scientists have launched laser beams at a reflector the size of a paperback novel about 240,000 miles (385,000 kilometers) away from Earth. They announced today, in collaboration with their French colleagues, that they received signal back for the first time

The idea of a human colony on Titan, a moon of Saturn, might sound crazy. Its temperature hovers at nearly 300° below zero Fahrenheit, and its skies rain methane and ethane that flow into hydrocarbon seas. Nevertheless, Titan could be the only place in the solar system where it makes sense to build a permanent, self-sufficient human settlement. //

But although the Moon and Mars look like comparatively reasonable destinations, they also have a deal-breaking problem. Neither is protected by a magnetosphere or atmosphere. Galactic Cosmic Rays, the energetic particles from distant supernovae, bombard the surfaces of the Moon and Mars, and people can’t live long-term under the assault of GCRs. //

GCRs include particles such as iron nuclei traveling at close to the speed of light that destroy brain tissue. //

On Earth, we are shielded from GCRs by water in the atmosphere. But it takes two meters of water to block half of the GCRs present in unprotected space. Practically, a Moon or Mars settlement would have to be built underground to be safe from this radiation.

Underground shelter is hard to build and not flexible or easy to expand. Settlers would need enormous excavations for room to supply all their needs for food, manufacturing and daily life. We ask why they would go to that trouble. We can live underground on Earth. What’s the advantage to doing so on Mars? //

Titan is the only other body in the solar system with liquid on the surface, with its lakes of methane and ethane that look startlingly like water bodies on Earth. It rains methane on Titan, occasionally filling swamps. Dunes of solid hydrocarbons look remarkably like Earth’s sand dunes.

For protection from radiation, Titan has a nitrogen atmosphere 50 percent thicker than Earth’s. Saturn’s magnetosphere also provides shelter. On the surface, vast quantities of hydrocarbons in solid and liquid form lie ready to be used for energy. Although the atmosphere lacks oxygen, water ice just below the surface could be used to provide oxygen for breathing and to combust hydrocarbons as fuel.

It’s cold on Titan, at -180°C (-291°F), but thanks to its thick atmosphere, residents wouldn’t need pressure suits—just warm clothing and respirators. Housing could be made of plastic produced from the unlimited resources harvested on the surface, and could consist of domes inflated by warm oxygen and nitrogen. The ease of construction would allow huge indoor spaces.

Titanians (as we call them) wouldn’t have to spend all their time inside. The recreational opportunities on Titan are unique. For example, you could fly. The weak gravity—similar to the Moon’s—combined with the thick atmosphere would allow individuals to aviate with wings on their backs. If the wings fall off, no worry, landing will be easy. Terminal velocity on Titan is a tenth that found on the Earth.

An asteroid crashing into the Moon could explain what the monks saw in 1178.

Astronomers have revealed the atmosphere of the red supergiant star Antares in stunning, unprecedented detail.

Click on satellites for more information about them. Click and drag to rotate the view.

All magnitudes and sky positions are shown for Texarkana. Change location.

A planisphere is a simple hand-held device which shows a map of which stars are visible in the night sky at any particular time. By rotating a wheel, it shows how stars move across the sky through the night, and how different constellations are visible at different times of year.

On this web page, I present a kit which you can download and print to make your own planisphere out of paper or cardboard.

The charts above show the division of the night sky into 88 constellations, plotted on a rectangular grid of right ascension and declination. They were generated using StarCharter, a command-line tool for producing vector-graphics charts of the night sky, written by the author and freely available for download.

To view the whole sky, click on Expand chart, or use the Download PDF link for the highest possible quality

Just over three years ago, because the Sun was temporarily blocked by the body of Saturn, the robotic Cassini spacecraft was able to look toward the inner Solar System. There, it spotted our Earth and Moon -- just pin-pricks of light lying about 1.4 billion kilometers distant. Toward the right of the featured image is Saturn's A ring, 

Still bathed in sunlight, the International Space Station arced through the evening sky over lake Wulfsahl-Gusborn in northern Germany, just after sunset on March 25. The familiar constellation of Orion can be seen left of the trail of the orbital station's bright passage. On the right, Venus is the brilliant evening star above the western horizon.

: It was bright and green and stretched across the sky. This striking aurora display was captured in 2016 just outside of Östersund, Sweden. Six photographic fields were merged to create the featured panorama spanning almost 180 degrees. Particularly striking aspects of this aurora include its sweeping arc-like shape and its stark definition. Lake Storsjön is seen in the foreground, while several familiar constellations and the star Polaris are visible through the aurora, far in the background. Coincidently, the aurora appears to avoid the Moon visible on the lower left. The aurora appeared a day after a large hole opened in the Sun's corona allowing particularly energetic particles to flow out into the Solar System. The green color of the aurora is caused by oxygen atoms recombining with ambient electrons high in the Earth's atmosphere.

Green Flashes: Sun, Moon, Venus, Mercury

Follow a sunset on a clear day against a distant horizon and you might glimpse green just as the Sun disappears from view. The green flash is caused by refraction of light rays traveling to the eye over a long path through the atmosphere. Shorter wavelengths refract more strongly than longer redder wavelengths and the separation of colors lends a green hue to the last visible vestige of the solar disk.

What if there was a lake on the Moon? What would it be like to swim in it? Presuming that it is sheltered in a regular atmosphere, in some giant dome or something. -- Kim Holder

This would be so cool.

In fact, I honestly think it's cool enough that it gives us a pretty good reason to go to the Moon in the first place. At the very least, it's better than the one Kennedy gave.

Floating would feel about the same on the Moon as on Earth, since how high in the water you float depends only on your body's density compared to the water's, not the strength of gravity.

Swimming underwater would also feel pretty similar. The inertia of the water is the main source of drag when swimming, and inertia is a property of matter[1] independent of gravity. The top speed of a submerged swimmer would be about the same on the Moon as here—about 2 meters/second.

Everything else would be different and way cooler. The waves would be bigger, the splash fights more intense, and swimmers would be able to jump out of the water like dolphins. //

but the bottom line is that a normal swimmer on the Moon could probably launch themselves a full meter out of the water, and Michael Phelps may well be able to manage 2 or 3. //

But it gets even better. A 2012 paper in PLoS ONE, titled Humans Running in Place on Water at Simulated Reduced Gravity, concluded that while humans can't run on the surface of water on Earth,[5] they might just barely be able to do so on the Moon. (I highly recommend reading their paper, if only for the hilarious experimental setup illustration on page 2.)

The bright parts around the black hole are the accretion disk, which is in reality just a flat disk in the equatorial plane similar to the rings of Saturn, but is distorted visually by gravitational lensing. You can see a page here that gives some code for creating images using ray-tracing of light rays in curved spacetime, which offers a more schematic diagram of the visual appearance of a disk around a black hole (with a checkerboard pattern on it for clarity):

[image]

In this Q&A with Kip Thorne, he gives some background on how they created the images, indicating that they used a more sophisticated technique than ray-tracing:

I had been seen many years ago an image of an accretion disk with gravitational lensing that Jean-Pierre Luminet in France had made. I had sort of forgotten about it, but when I first saw the gravitationally lensed accretion disk that you actually see in the movie, it was a mixture of amazement on one hand and recognition that “Yes I do remember seeing something like that, years ago.” And a bit of awe and excitement that this team at Double Negative had just taken the equations I had given them — they don’t just use ray tracing, they propagate ray bundles or light beams — they’d used light beam propagation equations, laid down their own accretion disk based on artistic models based on astrophysicist’s stuff, and come back to me with a full-blown image of the sort you see in the movie. I was really impressed and gratified that they pulled it off and was so pleased with how it looked.

They didn't simulate all the optical effects that would be seen though--the physicist mentioned above, Jean-Pierre Luminet, comments in a facebook post here that the Interstellar image doesn't include "the strong Doppler and gravitational spectral shifts induced by the rotation of the disk at relativistic speed", and that after commenting about this he got a message from Kip Thorne saying that "The doppler shift was left out of the images, because (as you showed long ago) it makes the disk highly asymmetric, and much harder for a mass audience to grasp."