Clear Sky Chart

Lore & Background

In 2000, Allan Rahill, a meteorologist at the Canadian Meteorological Centre and amateur astronomer, created a forecast processing step using data from CMC's Global Element Multi-scale (GEM) model to forecast cloud cover, specially designed to consider cirrus clouds. Later, Rahill added forecasts of astronomical transparency and astronomical seeing. In 2001, Attilla Danko began summarizing Rahill's hundreds of forecast maps by displaying one pixel from each map in rows, creating a meteogram called a clear sky chart for a single location. Danko accepts requests to create new charts for locations not currently covered, but since the GEM model only covers North America, charts are limited to that region. In 2020, Danko added Norwegian/European forecast (ECMWF) information to some charts, with research continuing on which forecast is more reliable.

Reader's Guide

The Clear Sky Chart is significant for amateur astronomy because it provides specialized forecasts—cloud cover, transparency, and seeing—that civil and aviation forecasts do not offer. Its creation by a meteorologist and a programmer at the Canadian Meteorological Centre leveraged the GEM model to address astronomers' needs, particularly cirrus cloud detection. The chart's point-forecast nature (9-mile radius) and 48-hour horizon make it practical for planning observing sessions. Its recognition includes awards from the Astronomical Society of the Pacific (2010), the Canadian Meteorological and Oceanographic Society (2004), the Royal Astronomical Society of Canada (2005), and the Astronomical League (2003). Asteroids were named after both creators. The name change in 2008 from 'Clear Sky Clock' to 'Clear Sky Chart' avoided potential legal action from Skyclock company. Alternative services include 7timer and Astrospheric. The charts are widely used on private and club astronomy websites in North America due to permissive terms of use for non-commercial sites.

Did You Know?

• Clear Sky Charts were originally called 'Clear Sky Clocks' until February 29, 2008.
• The charts are limited to a 9-mile radius and forecast at most 48 hours into the future.
• Allan Rahill's cloud forecast specially models cirrus clouds, which civil forecasts often call 'clear'.
• Asteroids 161693 Attilladanko and 171153 Allanrahill were named after the creators.

The Spirit of the Hobby and Its Practitioners

Amateur astronomy occupies a unique space in the world of sky observation. Unlike professional astronomers, participants in this field do not earn their livelihood from the discipline, nor do they typically hold advanced degrees in astrophysics. Yet this lack of formal institutional affiliation does not diminish their contributions. Many hobbyists monitor variable stars, track sunspots, record occultations of stars by the Moon or asteroids, and even discover transient events like comets, novae, or supernovae in distant galaxies. While the primary motivation is passion rather than publication, a meaningful subset of amateurs work alongside professionals, lending their observational skills to broader research efforts. The boundary between amateur and professional astronomy, once blurred throughout most of human history, solidified in the early twentieth century as the field became a clearly delineated academic and career pursuit. Today, the hobby spans a wide spectrum: from casual stargazers who glance up with the naked eye to highly experienced observers who run private or club observatories and assist in formal research programs.

From Naked Eye to Radio Waves — The Toolkit

The equipment landscape in amateur astronomy is remarkably broad, stretching from the simplest tools to sophisticated instruments that rival some professional setups. At the most basic level, an observer needs nothing more than their own eyes or a pair of binoculars, which offer a wider field of view and greater portability than most telescopes. Moving up, portable optical telescopes of varying power and quality form the backbone of many observers' setups, while more dedicated individuals operate instruments housed in private or club observatories. Astrophotography has surged in popularity as digital cameras, DSLRs, and purpose-built CCD and CMOS sensors have made capturing the night sky far more accessible. Beyond the visible spectrum, a small but dedicated minority venture into non-visual observation. Grote Reber, an amateur in the late 1930s, built the first purpose-built radio telescope to follow up Karl Jansky's discovery of radio emissions from space. Today, some amateurs use infrared filters on conventional scopes or operate radio telescopes—either home-built or repurposed from research institutions, such as the One-Mile Telescope. For locating targets, setting circles and fully automated GOTO mounts have transformed how observers point their instruments at specific celestial coordinates.

Roots in Ancient Skies and a Modern Renaissance

The tradition of watching the sky for practical and wonder-driven reasons stretches back to the earliest civilizations. Babylonian, Chaldean, and ancient Egyptian observers tracked celestial bodies with nothing but the naked eye and simple tools, using their observations to build calendars, predict eclipses, and separate day from night in ways that directly informed agricultural planning. For millennia, this kind of sky-watching was simply how humanity understood the heavens, with no formal distinction between the observer's status and their purpose. The modern identity of amateur astronomy as a recognizable hobby crystallized in the mid-nineteenth century, when the practice attracted millions of participants. Early on, the activity was dominated by wealthy Grand Amateurs who could afford the best available instruments. A dramatic shift came after World War II, when manufacturing advances made telescopes and other optical equipment far more affordable and accessible to ordinary people. This democratization transformed the hobby from an elite pastime into a widespread global community, laying the groundwork for the diverse and inclusive field of sky observation we see today.

Community, Public Outreach, and the Fight Against Sky Glow

Amateur astronomy is as much a social and educational endeavor as it is a technical one. Observers commonly join local astronomical societies that provide guidance on finding and studying celestial objects, offer educational programs, and work to promote the science of astronomy to the broader public. One particularly engaging form of outreach is sidewalk astronomy, where observers set up telescopes in public spaces for casual viewing. This practice not only sparks curiosity about the night sky and science in general but also draws attention to the growing problem of light pollution. That problem is a real and escalating challenge: artificial sky glow washes out natural darkness and significantly reduces the visibility of faint celestial objects, making observation increasingly difficult for anyone who steps outside a city. To counter this, many dedicated observers travel to rural areas where the sky remains dark enough to reveal deep-sky targets like distant galaxies, nebulae, and star clusters. Whether through formal society meetings, public telescope nights, or simply sharing a view of the Moon with a neighbor, the community dimension of the hobby ensures that the joy of looking up continues to spread well beyond the small group of experienced observers.