240NAVIGATIONALASTRONOMYPOLARISVEGAMAD2102AD14000AXISOFEARTHPRECESSIONNUTATION23.5*GRAVITATIONAIATTRACTION1PLANEOF ECLIPTICROTATPLANE OF EQUINOCTIALFORCETENDING TO ROTATEEARTH'SAXISPERPENDICULARTO PLANE OF ECLIPTIC.-AD14000AD2102Figure1519c.Precessionandnutationhours,eachhourhaving60minutesof 60seconds,theversal Time if the Greenwichmeridian is usedlengthofeachoftheseunitsdifferssomewhat inthevariousTheperiodfromonevernal equinoxtothenext(thecy-kindsoftimecle of the seasons)is known as the tropical year.It isTime is also classified accordingtothe terrestrial me-approximately365 days, 5 hours, 48 minutes,45 secondsridian used as a reference.Local time results if one's ownthough the length has been slowly changing for many cen-meridian is used, zone time if a nearby reference meridianturies.Our calendar, the Gregorian calendar,approximatesthetropical yearwithacombination of commonyearsofis used over a spread oflongitudes, and Greenwich or Uni-
240 NAVIGATIONAL ASTRONOMY hours, each hour having 60 minutes of 60 seconds, the length of each of these units differs somewhat in the various kinds of time. Time is also classified according to the terrestrial meridian used as a reference. Local time results if one’s own meridian is used, zone time if a nearby reference meridian is used over a spread of longitudes, and Greenwich or Universal Time if the Greenwich meridian is used. The period from one vernal equinox to the next (the cycle of the seasons) is known as the tropical year. It is approximately 365 days, 5 hours, 48 minutes, 45 seconds, though the length has been slowly changing for many centuries. Our calendar, the Gregorian calendar, approximates the tropical year with a combination of common years of Figure 1519c. Precession and nutation
241NAVIGATIONALASTRONOMYmLeoVirgo3Libraalnb2T*wPiscesAqaariusFigure1520.Thezodiac.365daysand leapyearsof366days.Aleapyear isanyyearmoon's apparent diameter is larger than that of the sun, anddivisible byfour, unless it is a century year, which must beits shadow reaches the earth as a nearlyround dot onlyadivisible by 400 to be a leap year.Thus,1700,1800,andfew miles in diameter.The dot moves rapidly across the1900werenot leapyears,but2000will be.Acritical mis-earth,from westto east,as themoon continues in itsorbittakewasmadebyJohnHamiltonMooreincalling1800aWithinthedot,thesun is completelyhiddenfrom view,andleap year, causing an error in thetables in his book,Theatotaleclipseofthesunoccurs.ForaconsiderabledistancePractical Navigator.This error caused the loss of at leastaround the shadow, part of the surface of the sun is ob-oneshipandwaslaterdiscoveredbyNathanielBowditchscured, and a partial eclipse occurs.In the line of travel ofwhilewritingthefirsteditionof TheNewAmericanPracti-the shadowapartial eclipse occurs as the round disk of thecal Navigator.moonappearstomoveslowlyacrossthesurfaceofthesun,SeeChapter18foran in-depthdiscussionoftimehiding an ever-increasing part of it, until the total eclipseoccurs.Because of the uneven edge of the mountainous1522.Eclipsesmoon, the light is not cut off evenly. But several last illumi-nated portions appear throughthevalleys orpasses betweenthe mountain peaks.These are called Baily's Beads.A totalIf theorbit of themoon coincidedwiththeplaneof theeclipse is a spectacularphenomenon.As thelast lightfromecliptic, themoonwouldpass in front of the sun at everythesuniscutoffthesolarcorona.orenvelopeofthin.ilnewmoon,causingasolareclipse.Atfullmoon,themoonluminatedgasaroundthesunbecomesvisible.Wispsofwould passthrough the earth's shadow,causing a lunarmore dense gas may appear as solar prominences.Theeclipse.Becauseofthemoon's orbitisinclined5°withre-only light reaching the observer is that diffused by the at-spectto the ecliptic,themoonusuallypasses above orbelowmosphere surroundingthe shadow.As themoonappears tothe sun at newmoon and aboveorbelowthe earth'sshadowcontinue on acrossthefaceof the sun,the sunfinallyatfull moon.However,there aretwopointsatwhichtheemergesfrom the otherside,firstas Baily's Beads,and thenplane of the moon's orbit intersects the ecliptic.These arethenodes of themoon's orbit.If themoonpasses oneofas an everwidening crescentuntil nopart of its surfaceisobscuredbythemoon.these points at the same time as the sun, a solar eclipse takesplace.This is shown in Figure 1522Theduration ofa total eclipsedepends upon how near-The sun andmoon are of nearly the sameapparentsizelythemoon crossesthe centerofthesun,thelocation oftheshadow on the earth,the relativeorbital speeds of themoonto an observer on the earth.If themoon is at perigee, the
NAVIGATIONAL ASTRONOMY 241 365 days and leap years of 366 days. A leap year is any year divisible by four, unless it is a century year, which must be divisible by 400 to be a leap year. Thus, 1700, 1800, and 1900 were not leap years, but 2000 will be. A critical mistake was made by John Hamilton Moore in calling 1800 a leap year, causing an error in the tables in his book, The Practical Navigator. This error caused the loss of at least one ship and was later discovered by Nathaniel Bowditch while writing the first edition of The New American Practical Navigator. See Chapter 18 for an in-depth discussion of time. 1522. Eclipses If the orbit of the moon coincided with the plane of the ecliptic, the moon would pass in front of the sun at every new moon, causing a solar eclipse. At full moon, the moon would pass through the earth’s shadow, causing a lunar eclipse. Because of the moon’s orbit is inclined 5° with respect to the ecliptic, the moon usually passes above or below the sun at new moon and above or below the earth’s shadow at full moon. However, there are two points at which the plane of the moon’s orbit intersects the ecliptic. These are the nodes of the moon’s orbit. If the moon passes one of these points at the same time as the sun, a solar eclipse takes place. This is shown in Figure 1522. The sun and moon are of nearly the same apparent size to an observer on the earth. If the moon is at perigee, the moon’s apparent diameter is larger than that of the sun, and its shadow reaches the earth as a nearly round dot only a few miles in diameter. The dot moves rapidly across the earth, from west to east, as the moon continues in its orbit. Within the dot, the sun is completely hidden from view, and a total eclipse of the sun occurs. For a considerable distance around the shadow, part of the surface of the sun is obscured, and a partial eclipse occurs. In the line of travel of the shadow a partial eclipse occurs as the round disk of the moon appears to move slowly across the surface of the sun, hiding an ever-increasing part of it, until the total eclipse occurs. Because of the uneven edge of the mountainous moon, the light is not cut off evenly. But several last illuminated portions appear through the valleys or passes between the mountain peaks. These are called Baily’s Beads. A total eclipse is a spectacular phenomenon. As the last light from the sun is cut off, the solar corona, or envelope of thin, illuminated gas around the sun becomes visible. Wisps of more dense gas may appear as solar prominences. The only light reaching the observer is that diffused by the atmosphere surrounding the shadow. As the moon appears to continue on across the face of the sun, the sun finally emerges from the other side, first as Baily’s Beads, and then as an ever widening crescent until no part of its surface is obscured by the moon. The duration of a total eclipse depends upon how nearly the moon crosses the center of the sun, the location of the shadow on the earth, the relative orbital speeds of the moon Figure 1520. The zodiac