Astronomy - Observing the Sky - Quick Guides | Astronomy

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Study GuideAstronomy–Observing the Sky1.Naked-Eye AstronomyHuman understanding of the universe began when people started observing the sky with their owneyes. Long before telescopes existed, people noticed that objects in the sky changed their positionsover days, months, and years. These careful observations laid the foundation of astronomy.1.1The Sky as We See ItWhen we look up from Earth, the sky appears like a huge dome or bowl covering us from all sides.Astronomers call this imaginary dome thecelestial sphere.•Thehorizonis the circle where the sky seems to meet the ground.•Thezenithis the point directly overhead.Because stars and other objects are extremely far away, our eyes cannot judge their distances. As aresult, all objects appear to lie on the same spherical surface.1.2The Horizon (Alt-Azimuth) Coordinate SystemTo locate objects in the sky, astronomers use coordinates, just like latitude and longitude on Earth.•Altitude: how high an object is above the horizon•Azimuth: the direction of the object, measured along the horizon starting from due north andmoving eastwardThis method is called thehorizon coordinate systemoralt-azimuth system.

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Study Guide1.3Stars and ConstellaƟonsOn a very dark night, about6,000 starscan be seen with the naked eye. The relative positions ofstars remain almost unchanged over human lifetimes.To make sense of the sky, people grouped bright stars into patterns calledconstellations. Thesepatterns often resemble animals, objects, or mythological figures.•Northern Hemisphere constellations, such asUrsa Major (Big Bear)andOrion, havebeen known since ancient times.•Southern Hemisphere constellationswere added later to cover areas not visible from thenorth.Today, the sky is officially divided into88 constellations.Naming StarsStars are named using:•AGreek letter(to indicate brightness)•Thename of the constellationFor example:•αUrsae Majoris (αUMa)is the brightest star in Ursa Major•It is also known by its ancient name,Dubhe

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Study Guide1.4Daily MoƟon of the StarsAs Earth rotates, stars appear to move across the sky fromeast to west.•Stars seem to move in circular paths around a point called thecelestial pole•In the Northern Hemisphere, this point is marked byPolaris (the Pole Star)Thealtitude of Polarisabove the horizon equals theobserver’s latitudeon Earth.1.5Circumpolar and Equatorial Stars•Circumpolar starsnever set; they stay above the horizon all night.•Equatorial starsrise in the east, reach their highest point on themeridian, and set in thewest.Themeridianis an imaginary line that runs:•From north on the horizon•Through the zenith•To south on the horizon1.6The CelesƟal Equator and Cardinal DirecƟonsThecelestial equatoris Earth’s equator projected onto the sky. It crosses the horizon exactly ateastand west.This helps define thecardinal directions:•North•South•East•WestThese directions are essential for orientation on Earth and in the sky.

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Study Guide1.7The Equatorial Coordinate SystemBecause altitude and azimuth change constantly, astronomers use a fixed coordinate system tied tothe stars.This is theequatorial coordinate system, which is similar to Earth’s latitude and longitude.•Declination→celestial latitude (north or south of the celestial equator)•Right Ascension (RA)→celestial longitudeDeclination is measured in degrees, minutes, and seconds.Right ascension is measured inhours, with24 hours = 360°.1.8The Sun’s MoƟon in the SkyLike stars, the Sun rises in the east and sets in the west. However, its path changes throughout theyear.Equinoxes•Vernal (Spring) Equinox–aroundMarch 21•Autumnal Equinox–aroundSeptember 21On these days:•The Sun lies on the celestial equator•Day and night are equal in length

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