- APOD — Astronomy Picture of the Day - each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
- Astrophysical Data System — The SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant.
- Space.com — a source for news of astronomy, skywatching, space exploration, commercial spaceflight and related technologies.
- Google.com/mars — Like Google Earth, but for the red planet, really cool.
- Students for the Exploration and Development of Space — gateway to many sources, including the Messier Catalog.
- The Net Advance of Physics: Astronomy and Astrophysics— lecture notes and more at various levels.
- NASA.gov — NASA's official website
- Webstars — Astronomy Resources for Educators, Students, and the Public at the HEASARC. The HEASARC provides a number of facilities for interested audiences - students, educators, and the general public - to learn more about the universe.
- Basics of Radio Astronomy
- Infrared Astronomy — from NASA’s Infrared Processing and Analysis Center.
- U. S. Geological Survey Astrogeology Research Program — Many high-quality maps of Mars and other planets and provides useful information about our solar system.
- SkyView — a Virtual Observatory on the Net generating images of any part of the sky at wavelengths in all regimes from Radio to Gamma-Ray.
- High Energy Astrophysics Science Archive Research Center (HEASARC) — images, descriptions, and data from x-ray and gamma-ray astronomy satellites. HEASARC is the primary archive for NASA's (and other space agencies') missions studying electromagnetic radiation from extremely energetic cosmic phenomena ranging from black holes to the Big Bang.
- Heavens Above — Gives up-to-date info on the International Space Station, like when it will be over your location. Also get star charts, information on the Solar System and more.
- Hubble Site — A site for The Hubble Space Telescope that features awesome pictures it's taken.
- The James Webb Space Telescope — Plans for the Hubble’s successor are underway. The James Webb Space Telescope (sometimes called JWST or Webb) will be a large infrared telescope with a 6.5-meter primary mirror. The telescope will be launched on an Ariane 5 rocket from French Guiana in Spring 2019.
- The Solar System — JPL site with links to missions. See also The Nine Planets by Bill Arnett and (if you can stand the ads) Views of the Solar System by C.J. Hamilton.
- Space Physics at the National Space Sciences Data Center — links to details of past, present, and future Solar System exploration missions.
- Galileo explored Jupiter and its satellites from 1995 to 2003
- The Exploration of the Earth’s Magnetosphere — a non-mathematical but quite detailed overview of space research on the Earth’s environment in space (material is being kept online for archival purposes only).
- Apollo Lunar Surface Journal — Transcripts, commentary, and spectacular images from when astronauts walked on the Moon. See the images from Apollo 17.
- Lunar panoramas from Apollo — Hans Nyberg has spliced together photographs to make 360° panoramas accompanied by audio recordings of astronauts at the time.
- Minor Planets (asteroids) — see also the list by number (The official discovery circumstances for the numbered minor planets are maintained by the Minor Planet Center.) or by name.
- NEAR Shoemaker satellite orbited asteroid Eros for a year, landed 12 February 2001.
- Was there once life on Mars? — about a controversial meteorite.
- Meteorite Central and Meteorites and Their Properties — two sites on those rocks from out of this world.
- Kuiper Belt Objects — all about those trans-Neptunian objects by David Jewitt, one of the leaders in discovering and interpreting them.
- The Extrasolar Planets Encyclopaedia — all the news about planets orbiting other suns.
- California & Carnegie Planet Search — from the leading American researchers in the field. an interactive table and plotter for exploring and displaying data from the Exoplanet Orbit Database.
- Sky & Telescope — A guide to astronomy, offers fascinating facts on space and astronomy and is excellent for amateur astronomers.
- The Search for Extraterrestrial Life — much information and many links, provided by the Planetary Society. The past, present and future of SETI, the search for extraterrestrial intelligence.
- The Sun — All about our sun
- Cambridge Cosmology — includes animations, brief history of 20th century cosmology.
- Space Daily — Self-described as your portal to space. A great place for space-related news.
- EarthSky - Updates on your cosmos and world.
Institutions, Organizations, Observatories
- AAO ESO Gemini HST NOAO NRAO SDSS SOHO Subaru Yohkoh
- NASA, including Astrobiology Institute Goddard HST JPL All science missions (past, present, & future)
- American Association of Variable Star Observers (AAVSO)
- American Astronomical Society (AAS) — the leading professional society in the USA.
- Association of Universities for Research in Astronomy
- Astronomical Society of the Pacific (ASP) — for professionals, amateurs, teachers, and others. Advancing science literacy through astronomy.
- Berkeley Cosmology Group — good material on dark matter, cosmology. They bring together a wide array of talents and techniques to address topics ranging from cosmological probes of fundamental physics to the formation and evolution of galaxies.
- College Departments Offering Astronomy Related Degrees — a list of Astronomy degree granting Institutions.
- Smithsonian National Air and Space Museum.
- International Astronomical Union (IAU) — the leading professional society on Earth. founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation.
- International Dark Sky Association — dedicated to sound lighting practices and preserving dark skies, and works to protect the night skies for present and future generations.
- The Meteoritical Society — to promote the study of extraterrestrial materials and their history.
- The National Space Society — promotes change in social, technical, economic, and political conditions to advance the day when people will live and work in space. The Society publishes Ad Astra magazine and maintains an active global network of volunteers and local chapters. Membership and participation are open to all.
- The Planetary Society— founded by Carl Sagan and Bruce Murray.
- The Royal Astronomical Society — in London since 1820.
- The Royal Astronomical Society of Canada — includes both professionals and amateurs. RASC is a national, non-profit, charitable organization devoted to the advancement of astronomy and related sciences.
- The SETI Institute — now privately funded, the search for extraterrestrial life goes on.
Astronomy as a Hobby
- Amateur Astronomy Clubs and Organizations — maintained by the ASP.
- Astronomy for Beginners: How to Get Started in Backyard Astronomy — by Alan MacRobert
- The Antique Telescope Society — Site includes pictures of wonderful old telescopes.
- Deep Sky Astronomy — Observing notes, sketches, and more from Emil Neata in Romania.
- The Stellafane Home Page — the Springfield Telescope Makers.
- How do I start with astronomy as a hobby? — some good answers on Quora
- How Telescopes Work — by Craig C. Freudenrich.
- How to Start Right in Astronomy — By Alan MacRobert on Sky and Telescope
- How to Become a Backyard Astronomer — By Steve Rousseau on Popular Mechanic
Eight Lunar Phases
Lunar phases refer to the shapes of the illuminated portion of the Moon. These shapes are caused by the positions of the Moon relative to the Sun as viewed by observers on Earth.
There are four main phases and four intermediate phases of the moon. Each phase refers to the amount of illumination the Sun casts on the lunar surface.
The four main phases of our Moon are new moon, first quarter, full moon, and third quarter.
In all phases. View assumed from the center of the earth.
(Geometrically impossible, but best demonstration of
monthly libration caused by moon's motion By Tomruen
[Public domain], via Wikimedia Commons