Transcript Slide 1
By What is an eclipse? • A solar eclipse occurs when the Moon comes between the Sun and the observer. • A lunar eclipse occurs when Earth comes between the Sun and the Moon. • A total eclipse happens when the moon entirely blocks the Sun or the Earth's shadow totally blocks our view of the moon • An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. •A hybrid eclipse is intermediate between a total and annular eclipse. At some points on the surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Phases of the Moon • Eclipses will occur during some of these phases. • There are many kinds of eclipse, two of which are solar and lunar eclipses. Solar Eclipses Solar Eclipses • Occur at New Moon because we will not be able to see the moon due to the illuminated side of the moon being pointed away from Earth. • If the moon’s shadow happens to fall upon Earth’s surface at that time, we will see some portion of the sun’s disk covered or ‘eclipsed’ by the moon. Lunar Eclipses • Earth’s shadow is composed of two cone – shaped components, one inside the other. Lunar eclipses • Everyone on the night side of Earth can see an eclipse of the Moon when it takes place • Lunar eclipses can be useful because you can measure the exact time when each crater of the moon enters and leaves the umbral shadow. • These crater timings can be used to estimate the enlargement of the earth’s atmosphere due to airborne dust and volcanic ash. Why don’t they occur every month when the moon is in the full moon phase? • The Moon’s orbit about Earth is tilted 5 degrees to Earth’s orbit around the sun. • Meaning the moon spends most of the time either above or below the plane of earth’s orbit. • The plane of Earth’s orbit around the Sun is important because Earth’s shadows lie in exactly the same place. • During Full Moon, the Moon usually passes above or below earth’s shadows and misses them entirely so no eclipse takes place. Partial and Total Lunar Eclipses • From 2000BC through 3000 AD, there are 7,718 eclipses of the Moon • There are anywhere from 0 to 3 lunar eclipses each year. • The last time that three total lunar eclipses occurred in one calendar year was in 1982. • Partial eclipses slightly outnumber total eclipses by 7 to 6. Partial and Total Solar Eclipses Year Total Partial Annular Hybrid 1998 1 (Feb) 0 1 (Aug) 0 1999 1 (Aug) 0 1 (Feb) 0 2000 0 5 (Feb, July (x2), Dec) 0 0 2001 1 (June) 0 1 (Dec) 0 2002 1 (Nov) 0 1 (June) 0 2003 1 (Dec) 0 1 (May) 0 2004 0 2 (Apr, Oct) 0 0 2005 0 0 1 (Apr) 1 (Apr) 2006 1 (Mar) 0 1 (Sept) 0 2007 0 2 (Mar, Sept) 0 0 2008 0 0 1 (Feb) 0 2009 2 (July & Aug) 0 1 (Jan) 0 Total Eclipse in Cornwall 11/08/1999 •The last total eclipse in Britain prior to this, was over 80 years ago, for us in Britain this is a once in a lifetime experience. •The partial eclipse started at about 10 a.m., and by 11.11 it was total darkness, and returns to partial eclipse approximately 2 minutes later, before full sunlight is restored around noon Binary Stars • A binary star system consists of 2 stars that orbit around a common point • Four types of binary system – – – – Visual binaries Spectroscopic binaries Eclipsing binaries X-ray binaries • 5-10% of visible stars are visual binaries • ~50% of all stars are members of binary systems Background image: Artists impression of binary system called HD 113766 Visual and Spectroscopic Binaries • Visual binaries: – Individual stars visible through a telescope • Spectroscopic binaries: – Stars are so close together they appear as a single star – Binary nature deduced from their periodic Doppler shifts of wavelengths of lines seen in spectrum – Double-lined spectroscopic binaries – spectrum shows lines from both stars – Single-lined spectroscopic binaries – only one set of lines seen, other is too faint Eclipsing and X-ray Binaries • Eclipsing binaries: – Orbital plane is oriented exactly edgewise to the plane of the sky so one star passes directly in front of the other, blocking out its line during the eclipse – May also be visual or spectroscopic binaries • X-ray binaries: – Emit x-rays – Made up of normal star and collapsed star – These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. Transits • Occurs when, for example, a planet passes across the disk of the Sun. • On Earth we only experience transits of Mercury and Venus. • Transits far rarer than eclipses of the Sun by the moon. • On average we experience 13 transits of Mercury each century. • Transits of Venus are much rarer • The first transit that was recorded was in 1631 by the French astronomer Gassendi. Transits of Mercury • Transits of Mercury occur within a few days of either 8th May or 10th November. • Transits will occur if Mercury passes into the path between the Earth and the Sun at this time. • Chances of a November transit occurring are almost twice as high as those for a May transit • The last transit of Mercury occurred on 7th May 2003, being the first one seen since 1999. Visibility of 7th May 2003 Transit of Mercury Transits of Venus • Only 6 transits of Venus since the invention of the telescope, occurring in 1631, 1639, 1761, 1769, 1874 and 1882. • Transits of Venus only occur in early December and June. • The last transit of Venus occurred on 8th June 2004 • The next will occur on 6th June 2012 Visibility of 8th June 2004 Transit of Venus