Astronomy, HER-IS
Human beings have long been fascinated by the celestial sphere above, whose twinkling lights have inspired not only scientific theories but also many artistic endeavors. Humankind's fascination with the world beyond Earth has led to many landmark moments in history, as when space exploration took a giant step forward with the advent of technology that allowed humans to successfully travel to the Moon and to build spacecraft capable of exploring the rest of the solar system and beyond.
Astronomy Encyclopedia Articles By Title
Hermes
Hermes, binary asteroid whose eccentric orbit takes it near Earth. It was discovered on October 28, 1937, by German......
Herschel
Herschel, European Space Agency space telescope, launched on May 14, 2009, that studied infrared radiation from......
Herschel, Caroline
Caroline Herschel was a German-born British astronomer who was a pioneer in the field and is considered the first......
Herschel, Sir John, 1st Baronet
Sir John Herschel, 1st Baronet was an English astronomer and successor to his father, Sir William Herschel, in......
Herschel, William
William Herschel was a German-born British astronomer, the founder of sidereal astronomy for the systematic observation......
Hertzsprung, Ejnar
Ejnar Hertzsprung was a Danish astronomer who classified types of stars by relating their colour to their absolute......
Hertzsprung-Russell diagram
Hertzsprung-Russell diagram, in astronomy, graph in which the absolute magnitudes (intrinsic brightness) of stars......
Hesperus
Hesperus, in Greco-Roman mythology, the evening star; although initially considered to be the son of Eos (the Dawn)......
Hevelius, Johannes
Johannes Hevelius was an astronomer who compiled an atlas of the Moon (Selenographia, published 1647) containing......
Hewish, Antony
Antony Hewish was a British astrophysicist who won the Nobel Prize for Physics in 1974 for his discovery of pulsars......
High Energy Transient Explorer-2
High Energy Transient Explorer-2 (HETE-2), international satellite designed to study gamma-ray bursts (GRBs), intense......
Hill, George William
George William Hill was an American mathematical astronomer considered by many of his peers to be the greatest......
Hinode
Hinode, a Japanese-U.S.-U.K. satellite that carried a 50-cm (20-inch) solar optical telescope, a 34-cm (13-inch)......
HIP 13044b
HIP 13044b, first extrasolar planet that was found orbiting a star that originated outside the Milky Way Galaxy.......
Hipparchus
Hipparchus was a Greek astronomer and mathematician who made fundamental contributions to the advancement of astronomy......
Hipparcos
Hipparcos, Earth-orbiting satellite launched by the European Space Agency in 1989 that over the next four years......
Hoba meteorite
Hoba meteorite, the largest intact meteorite, weighing about 60 tons, that has been found on Earth. It was discovered......
Hobby-Eberly Telescope
Hobby-Eberly Telescope (HET), telescope that is one of the largest in the world, with a mirror measuring 11.1 by......
Hoffman, Jeffrey
Jeffrey Hoffman is an American astronaut and professor at the Massachusetts Institute of Technology (MIT). Having......
Hohmann orbit
Hohmann orbit, most economical path (though not the shortest or fastest) for a spacecraft to take from one planet......
Hooke, Robert
Robert Hooke was an English physicist who discovered the law of elasticity, known as Hooke’s law, and who did research......
Horizon
Horizon, in astronomy, boundary where the sky seems to meet the ground or sea. (In astronomy it is defined as the......
Horologium
Horologium, constellation in the southern sky at about 3 hours right ascension and 50° south in declination. Its......
Horrocks, Jeremiah
Jeremiah Horrocks was a British astronomer and clergyman who applied Johannes Kepler’s laws of planetary motion......
Horsehead Nebula
Horsehead Nebula, (catalog number IC 434), ionized-hydrogen region in the constellation Orion. The nebula consists......
Hour angle
Hour angle, in astronomy, the angle between an observer’s meridian (a great circle passing over his head and through......
Hour circle
Hour circle, in astronomy, any great circle (similar to longitude) on the celestial sphere that passes through......
How big is Jupiter?
Jupiter is the largest planet in the solar system, with an equatorial diameter of about 143,000 kilometers (88,900......
How big is Mars?
Mars is a small planet, larger than only Mercury and slightly more than half the size of Earth. Its equatorial......
How big is Pluto?
Pluto’s diameter, 2,370 km (1,473 miles), is less than half that of Mercury (4,880 km [3,032 miles]) and only about......
How big is Saturn?
Saturn is a colossal planet, with an equatorial diameter of 120,536 kilometers (74,898 miles). Saturn is the second-largest......
How big is the Moon?
The Moon is a spherical rocky body with a mean equatorial radius of approximately 1,738 kilometers (1,080 miles)......
How big is the Sun?
The Sun’s radius is a staggering 109 times greater than Earth’s, making it a truly massive presence in our solar......
How big is Venus?
Venus, the second planet from the Sun, is quite similar to Earth in terms of size, mass, and density. In fact,......
How Big Was the Asteroid That Contributed to Dinosaur Extinction?
Many scientists believe an asteroid that struck Earth 66 million years ago may have significantly contributed to......
How Do Black Holes Really Work?
Science fiction has often relied on the concept of black holes as a plot device, painting them as portals to other......
How Do We Know How Far Away the Stars Are?
The closest star, Proxima Centauri, is 4.24 light-years away. A light-year is 9.44 trillion km, or 5.88 trillion......
How Do You Tell the Difference Between Total, Annular, Solar, and Lunar Eclipses?
Traditionally, eclipses are divided into two major types: solar and lunar. Solar eclipses occur when the Moon passes......
How does a telescope work?
Telescopes have upended astronomy again and again by illuminating what was previously invisible to the naked eye.......
How far is the Moon from Earth?
The Moon is approximately 384,000 km (238,600 miles) away from Earth, on average. The Moon orbits Earth in an elliptical......
How far is the Sun from Earth?
The Sun is approximately 149.6 million km (about 92.96 million miles) away from Earth. This distance is often expressed......
How Fast Is the Universe Expanding?
The rate at which the universe is expanding is called the Hubble constant, named after astronomer Edwin Hubble,......
How Gemini 8 Made History and Avoided Disaster
The Gemini spaceflights of 1965–66 were designed to acquire the skills necessary for the Apollo lunar landing program.......
How hot is the Sun?
The surface of the Sun, known as the photosphere, reaches a temperature of about 5,800 kelvin (K; 10,000 degrees......
How long does it take to get to the Moon?
The time taken to reach the Moon varies depending on propulsion technology, a spacecraft’s trajectory, and the......
How long would it take to get to Mars?
On foot, in a straight line? 1,065 years. By car? 40 years. If your preferred mode of travel is a spacecraft—perhaps......
How many moons does Earth have?
You may have seen headlines such as “NASA confirms Earth now has two moons until 2083.” But we first need to define......
How Many People Have Been to the Moon?
Even if you weren’t alive for the first Moon landing on July 20, 1969, you’ve probably seen footage of it, when......
How Much Trash Is In Space?
Space seems pretty empty. After all, much of the volume of the universe is a vacuum that will kill you. But humanity......
How often does a solar eclipse happen?
Solar eclipses occur two to five times a year, though it is rare for five to occur in a year. The last time five......
How One Shrimp Used Its Survival Skills to Become Both Best-Selling Pet and Astronaut
A few years after humans first went to the Moon, Sea-Monkeys joined them. Let’s get the disappointment out of the......
How to Watch an Eclipse Safely
Earth, the Sun, and the Moon are coming into alignment. A solar eclipse will be happening near you. What do you......
How was Earth created?
Earth’s creation is a fascinating tale of interstellar material coming together to form our planet. It all began......
How was gravity discovered?
Isaac Newton didn’t just see an apple fall from a tree. He saw a clue to how gravity works. In 1687, he proposed......
How was the universe created?
The most widely accepted explanation for the origin of the universe is the big-bang theory, which proposes that......
How will the Nancy Grace Roman Space Telescope discover new exoplanets?
The Nancy Grace Roman Space Telescope will help scientists determine how common Earth-like worlds are in our galaxy.......
How will the Roman Space Telescope learn about dark energy and dark matter?
The Nancy Grace Roman Space Telescope is designed to help astronomers answer questions about dark energy and dark......
Hoyle, Sir Fred
Sir Fred Hoyle was a British mathematician and astronomer best known as the foremost proponent and defender of......
HR 8799
HR 8799, star that has the first extrasolar planetary system to be seen directly in an astronomical image. HR 8799......
Hubble constant
Hubble constant, in cosmology, constant of proportionality in the relation between the velocities of remote galaxies......
Hubble Space Telescope
Hubble Space Telescope (HST), the first sophisticated optical observatory placed into orbit around Earth. Earth’s......
Hubble, Edwin
Edwin Hubble was an American astronomer who played a crucial role in establishing the field of extragalactic astronomy......
Huggins, William
William Huggins was an English astronomer who revolutionized observational astronomy by applying spectroscopic......
Huitzilopochtli
Huitzilopochtli, Aztec sun and war god, one of the two principal deities of Aztec religion, often represented in......
Hulse, Russell Alan
Russell Alan Hulse is an American physicist who in 1993 shared the Nobel Prize for Physics with his former teacher,......
Hulst, Hendrik Christoffel van de
Hendrik Christoffel van de Hulst was a Dutch astronomer who predicted theoretically the 21-cm (8.2-inch) radio......
Huygens, Christiaan
Christiaan Huygens was a Dutch mathematician, astronomer, and physicist, who founded the wave theory of light,......
Hyades
Hyades, in Greek mythology, daughters of the Titan Atlas and the Oceanid Aethra, the five (or more) sisters of......
Hyades
Hyades, cluster of several hundred stars in the zodiacal constellation Taurus. As seen from Earth, the bright star......
Hydra
Hydra, constellation in the southern sky that stretches from 8 to 15 hours right ascension and from about 5° north......
Hydrogen cloud
Hydrogen cloud, interstellar matter in which hydrogen is mostly neutral, rather than ionized or molecular. Most......
Hydrus
Hydrus, constellation in the southern sky at about 2 hours right ascension and 70° south in declination. Its brightest......
Hyperion
Hyperion, major moon of Saturn, notable in that it has no regular rotation period but tumbles in an apparently......
Iapetus
Iapetus, outermost of Saturn’s major regular moons, extraordinary because of its great contrast in surface brightness.......
Ibn al-Haytham
Ibn al-Haytham was a mathematician and astronomer who made significant contributions to the principles of optics......
ibn Tibbon, Jacob ben Machir
Jacob ben Machir ibn Tibbon was a French Jewish physician, translator, and astronomer whose work was utilized by......
Icarus
Icarus, an Apollo asteroid (one that passes inside Earth’s orbit). It was discovered on June 27, 1949, by German-born......
Impact event
Impact event, collision of astronomical objects. Most collisions involve asteroids, comets, or meteoroids colliding......
Indian Space Research Organisation
Indian Space Research Organisation (ISRO), Indian space agency, founded in 1969 to develop an independent Indian......
Indus
Indus, constellation in the southern sky at about 21 hours right ascension and 50° south in declination. Its brightest......
Infographic: Spacecraft of the Apollo Program
This infographic explains the spacecraft of the Apollo program. More detailed information and explanations appear......
Infrared Astronomical Satellite
Infrared Astronomical Satellite (IRAS), U.S.-U.K.-Netherlands satellite launched in 1983 that was the first space......
Infrared astronomy
Infrared astronomy, study of astronomical objects through observations of the infrared radiation that they emit.......
Infrared source
Infrared source, in astronomy, any of various celestial objects that radiate measurable quantities of energy in......
Infrared Space Observatory
Infrared Space Observatory (ISO), European Space Agency (ESA) satellite that observed astronomical sources of infrared......
Infrared telescope
Infrared telescope, instrument designed to detect and resolve infrared radiation from sources outside Earth’s atmosphere......
Ingress
Ingress, in astronomy, the apparent entrance of a smaller body upon the disk of a larger one as the smaller passes......
Integral
Integral, European Space Agency–Russian–U.S. satellite observatory designed to study gamma rays emitted from astronomical......
Intergalactic medium
Intergalactic medium, material found between galaxies and that mostly consists of hot, tenuous hydrogen gas. At......
International Astronomical Union
International Astronomical Union (IAU), senior body governing international professional astronomical activities......
International Space Station
International Space Station (ISS), space station assembled in low Earth orbit largely by the United States and......
International Ultraviolet Explorer
International Ultraviolet Explorer (IUE), astronomical research satellite built in the 1970s as a cooperative project......
Interplanetary medium
Interplanetary medium, thinly scattered matter that exists between the planets and other bodies of the solar system,......
Interstellar medium
Interstellar medium, region between the stars that contains vast, diffuse clouds of gases and minute solid particles.......
Interstellar object
Interstellar object, an object that has entered the solar system from interstellar space. Three such objects have......
Inti
Inti, in Inca religion, the sun god; he was believed to be the ancestor of the Incas. Inti was at the head of the......
Io
Io, innermost of the four large moons (Galilean satellites) discovered around Jupiter by the Italian astronomer......
Iron meteorite
Iron meteorite, any meteorite consisting mainly of iron, usually combined with small amounts of nickel. When such......
Irwin, James B.
James B. Irwin was an American astronaut, pilot of the Lunar Module on the Apollo 15 mission (July 26–August 7,......
Is Comet 3I/ATLAS an alien spacecraft?
If you don’t hang out in the wilder areas of YouTube or TikTok, you may be thinking, “Wait. What? Is this a thing?......