Free Republic

Explanation: What's that unusual spot on the Moon? It's the International Space Station. Using precise timing, the Earth-orbiting space platform was photographed in front of a partially lit gibbous Moon in 2019. The featured image was taken from Palo Alto, California, USA with an exposure time of only 1/667 of a second. In contrast, the duration of the transit of the ISS across the entire Moon was about half a second. A close inspection of this unusually crisp ISS silhouette will reveal the outlines of numerous solar panels and trusses. The bright crater Tycho is visible on the lower left,...

Explanation: Can you find the Rosette Nebula? The red flowery-looking nebula just above the image center may seem a good choice, but that's not it. The famous Rosette Nebula is really located on the lower right, here colored blue and white, and connected to the other nebulas by gold-colored filaments. Because the featured image of Rosette's field is so wide, and because of its deep red exposure, it seems to contain other flowers. Designated NGC 2237, the center of the Rosette Nebula is populated by the bright blue stars of open cluster NGC 2244, whose winds and energetic light are...

Explanation: APOD is 30 years old today. In celebration, today's picture uses past APODs as tiles arranged to create a single pixelated image that might remind you of one of the most well-known and evocative depictions of planet Earth's night sky. In fact, this Starry Night consists of 1,836 individual images contributed to APOD over the last 5 years in a mosaic of 32,232 tiles. Today, APOD would like to offer a sincere thank you to our contributors, volunteers, and readers. Over the last 30 years your continuing efforts have allowed us to enjoy, inspire, and share a discovery of...

Explanation: How different does sunset appear from Mars than from Earth? For comparison, two images of our common star were taken at sunset, one from Earth and one from Mars. These images were scaled to have the same angular width and are featured here side-by-side. A quick inspection will reveal that the Sun appears slightly smaller from Mars than from Earth. This makes sense since Mars is 50% further from the Sun than Earth. More striking, perhaps, is that the Martian sunset is noticeably bluer near the Sun than the typically orange colors near the setting Sun from Earth. The...

Explanation: What are all those streaks in the sky? A galaxy, many satellite trails, and a few meteors. First, far in the distance, the majestic band of our Milky Way Galaxy runs down the left. Mirroring it on the right are several parallel trails of Earth-orbiting Starlink satellites. Many fainter satellite trails also crisscross the image. The two short and bright streaks are meteors — likely members of the annual Eta Aquariids meteor shower. The planet Venus shines on the lower right. Venus and the satellites shine by reflected sunlight. The featured picture is a composite of exposures all taken...

Explanation: In this Hubble Space Telescope image the bright, spiky stars lie in the foreground toward the heroic northern constellation Perseus and well within our own Milky Way galaxy. In sharp focus beyond is UGC 2885, a giant spiral galaxy about 232 million light-years distant. Some 800,000 light-years across compared to the Milky Way's diameter of 100,000 light-years or so, it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. Part of an investigation to understand how galaxies can grow to such enormous sizes, UGC 2885 was also part of An Interesting Voyage...

Explanation: On April 20, 2023 the shadow of a New Moon raced across planet Earth's southern hemisphere. When viewed along a narrow path that mostly avoided landfall, the Moon in silhouette created a hybrid solar eclipse. Hybrid eclipses are rare and can be seen as a total eclipse or an annular "ring of fire" eclipse depending on the observer's position. Viewers of this much anticipated hybrid event were able to witness a total solar eclipse while anchored in the Indian Ocean near the centerline of the eclipse track off the coast of western Australia. This ship-borne image from renowned eclipse...

Explanation: Do you know the names of some of the brightest stars? It's likely that you do, even though some bright stars have names so old they date back to near the beginning of written language. Many world cultures have their own names for the brightest stars, and it is culturally and historically important to remember them. In the interest of clear global communication, however, the International Astronomical Union (IAU) has begun to designate standardized star names. Featured here in true color are the 25 brightest stars in the night sky, currently as seen by humans, coupled with their IAU-recognized...

Explanation: Do oceans under the ice of Saturn's moon Enceladus contain life? A reason to think so involves long features -- some dubbed tiger stripes -- that are known to be spewing ice from the moon's icy interior into space. These surface cracks create clouds of fine ice particles over the moon's South Pole and create Saturn's mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown in true color from a close flyby. The deep crevasses are partly shadowed. Why...

Explanation: Can you identify this celestial object? Likely not — because this is a discovery image. Massive stars forge heavy elements in their cores and, after a few million years, end their lives in powerful supernova explosions. These remnants cool relatively quickly and fade, making them difficult to detect. To uncover such faint, previously unknown supernova remnants, a dedicated group of amateur astrophotographers searched through sky surveys for possible supernova remnant candidates. The result: the first-ever image of supernova remnant G115.5+9.1 — named Scylla by its discoverers—glowing faintly in the constellation of the mythological King of Aethiopia: Cepheus. Emission from...

Explanation: From our vantage point in the Milky Way Galaxy, we see NGC 3344 face-on. Nearly 40,000 light-years across, the big, beautiful spiral galaxy is located just 20 million light-years away in the constellation of Leo Minor. This multi-color Hubble Space Telescope close-up of NGC 3344 includes remarkable details from near infrared to ultraviolet wavelengths. The frame extends some 15,000 light-years across the spiral's central regions. From the core outward, the galaxy's colors change from the yellowish light of old stars in the center to young blue star clusters and reddish star forming regions along the loose, fragmented spiral arms....

Explanation: On the Mars rover's mission Sol 46 or Earth date April 6, 2021, Perseverance held out a robotic arm to take its first selfie on Mars. The WATSON camera at the end of the arm was designed to take close-ups of Martian rocks and surface details though, and not a quick snap shot of friends and smiling faces. In the end, teamwork and weeks of planning on Mars time was required to program a complex series of exposures and camera motions to include Perseverance and its surroundings. The resulting 62 frames were composed into a detailed mosaic, one of...

Explanation: The bright clusters and nebulae of planet Earth's night sky are often given the names of flowers or insects, and its whopping 3 light-year wingspan, NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the central star of the planetary nebula is transforming into a white dwarf star, becoming exceptionally hot, and shining brightly in ultraviolet light. The central star is hidden from direct view by a torus of dust, but its energetic ultraviolet light ionizes atoms in the nebula. In this sharp, telescopic view composed with narrowband image data, the ionized hydrogen...

Explanation: Savudrija lighthouse shines along the coast near the northern end of the Istrian peninsula in this well-composed night skyscape. A navigational aid for sailors on the Adriatic Sea, the historic lighthouse was constructed in the early 19th century. But Polaris, an even older aid to navigation, shines in the sky above. Alpha star of the constellation Ursa Minor, Polaris is also known as the North Star. In this scene Polaris forms the shortest bright arc near the North Celestial Pole, the extension of Earth's axis of rotation into space. Of course, the North Celestial Pole lies exactly at the...

Explanation: Is the sky the same every night? No -- the night sky changes every night in many ways. To better explore how the night sky changes, the USA's NSF and DOE commissioned the Vera C. Rubin Observatory in Cerro Pachón, Chile. In final testing before routine operations, Rubin will begin to explore these nightly changes -- slight differences that can tell us much about our amazing universe and its surprising zoo of objects. With a mirror over 8 meters across, Rubin will continually reimage the entire visible sky every few nights to discover new supernovas, potentially dangerous asteroids, faint...

Explanation: Why would the sky glow like a giant repeating rainbow? Airglow. Now, air glows all of the time, but it is usually hard to see. A disturbance however -- like an approaching storm -- may cause noticeable rippling in the Earth's atmosphere. These gravity waves are oscillations in air analogous to those created when a rock is thrown in calm water. The long-duration exposure nearly along the vertical walls of airglow likely made the undulating structure particularly visible. OK, but where do the colors originate? The deep red glow likely originates from OH molecules about 87 kilometers high, excited...

Explanation: Wisps like this are all that remain visible of a Milky Way star. About 7,000 years ago that star exploded in a supernova, leaving the Veil Nebula. At the time, the expanding cloud was likely as bright as a crescent Moon, remaining visible for weeks to people living at the dawn of recorded history. Today, the resulting supernova remnant, also known as the Cygnus Loop, has faded and is now visible only through a small telescope directed toward the constellation of the Swan (Cygnus). The remaining Veil Nebula is physically huge, however, and even though it lies about 1,400...

Explanation: What's happening to this spiral galaxy? Although details remain uncertain, it surely has to do with an ongoing battle with its smaller galactic neighbor. The featured galaxy is labelled UGC 1810 by itself, but together with its collisional partner is known as Arp 273. The overall shape of UGC 1810 -- in particular its blue outer ring -- is likely a result of wild and violent gravitational interactions. This ring's blue color is caused by massive stars that are blue hot and have formed only in the past few million years. The inner galaxy appears older, redder, and threaded...

Explanation: On May 7, the Sun setting behind a church bell tower was captured in this filtered and manipulated digital skyscape from Ragusa, Sicily, planet Earth. In this version of the image the colors look bizarre. Still, an intriguing optical illusion known as an afterimage can help you experience the same scene with a more natural looking appearance. To try it, find the sunspots of active region AR4079 grouped near the bottom of the blue solar disk. Relax and stare at the dark sunspot group for about 30 seconds, then close your eyes or shift your gaze to a plain...

Explanation: This composite of images spaced a weather-permitting 5 to 9 days apart, from 2024 September 19 (top right) through 2025 May 18 (bottom left), faithfully traces ruddy-colored Mars as it makes a clockwise loop through the constellations Gemini and Cancer in planet Earth's night sky. You can connect the dots and dates with your cursor over the image, but be sure to check out this animation of the Red Planet's 2024/25 retrograde motion. Of course Mars didn't actually reverse the direction of its orbit. Instead, the apparent backwards motion with respect to the background stars is a reflection of...