Celestial Timekeeping

Sidereal Day and Solar Day Sidereal Day = The time it takes a STAR to circle the Earth once. Solar Day = The time it takes the SUN to circle the Earth once. Why are they different?

Sidereal Day and Solar Day Sidereal day covers 360º of rotation: 23 h 56 m 4.09 s Solar day covers ~361º of rotation 24 h ± ~25 s It varies throughout the year…

It varies throughout the year – WHY? Kepler’s second law…

When is a solar day the longest? A) January B) April C) July D) October E) Solar days are all the same – sidereal days vary throughout the year

Sidereal Month and Synodic Month Sidereal month = The time it takes the Moon to go 360º around Earth. 27⅓ days Synodic Month = The time from New Moon to New Moon. 29½ days

The Moon rotates on its axis once in a … A) Synodic month B) Sidereal month C) Both are correct

Important planetary positions… …you should know Planets outside Earth’s orbit All planets orbit in the same direction Planets inside Earth’s orbit

Mercury’s position in the sky at the same time night after night. You can see it moving around its orbit as it “catches up” to Earth.

Venus’s position in the sky at the same time night after night. Venus moves slower, around a larger orbit, than Mercury, so its path is longer

Just because Mercury/Venus go through inferior conjunction doesn’t mean they pass in front of the Sun.

“Transit” Transits of Mercury happen every 3 – 13 years. (May 9, 2016; Nov 11, 2019) Transits of Venus are much more rare… “Transit”

Transits of Venus: 1631, 1639, 1761, 1769, 1874, 1882, 2004, 2012 … 2117, …

Noon-to-noon (“solar day”) is not always exactly 24 hours Apparent Solar Time Noon-to-noon (“solar day”) is not always exactly 24 hours

So our clocks are set to “Mean Solar Time” (“mean” = “average”)

These are both “local times” People in Pueblo would have their clocks set differently from people in Alamosa. Not a problem when travel by wagon took at least a day…

But when the railroads shrunk the distances between towns, something had to be done!

Railroad companies agreed to break the U.S. into 4 zones, November 18, 1883 Railroad companies agreed to break the U.S. into 4 zones, setting all clocks in each zone to the same time International consensus for global time zones was reached in 1884

International Dateline MST = UT – 7 Universal Time International Dateline Daylight Saving Time

Calendars: Tied to the Equinox Egyptian: 4200 B.C. 365 days in a year Spring equinox drifted 1 day LATER every 4 years

Calendars: Tied to the Equinox Julian: Julius Caesar - 46 B.C. Every 4th year has 366 days Too much correction: In 46 B.C, Spring Equinox on March 24 By 1500’s, Spring Equinox on March 11

Calendars: Tied to the Equinox Gregorian: Pope Gregory XIII – 1582 1. Skipped from Oct. 4  Oct. 15 This set the spring equinox to March 21 2. Every 4th year has 366 days YES NO EXCEPT for centuries not divisible by 400: 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700

Date and Time of Spring Equinox on Consecutive Years 1992 20 08 48 UT dd hh mm 2092 19 14 32 1993 20 14 41 2093 19 20 33 1994 20 20 28 2094 20 02 20 1995 21 02 15 2095 20 08 14 1996 20 08 03 2096 19 14 02 1997 20 13 55 2097 19 19 47 1998 20 19 55 2098 20 01 39 1999 21 01 46 2099 20 07 17 2000 20 07 35 2100 20 13 03 2001 20 13 31 2101 20 18 55 2002 20 19 16 2102 21 00 35 2003 21 01 00 2103 21 06 22 2004 20 06 49 2104 20 12 13 2005 20 12 33 2105 20 18 05 2006 20 18 26 2106 21 00 03 2007 21 00 07 2107 21 05 49 2008 20 05 48 2108 20 11 38 2009 20 11 44 2109 20 17 35 2010 20 17 32 2110 20 23 21 2011 20 23 21 2111 21 05 10 2012 20 05 14 2112 20 11 02 2013 20 11 02 2113 20 16 50 2014 20 16 57 2114 20 22 38 2015 20 22 45 2115 21 04 21 2016 20 04 30 2116 20 10 08 Date and Time of Spring Equinox on Consecutive Years

Why do we have leap years? Because the Earth’s precession causes the spring equinox to drift by 6 hours every year. Because the Earth takes almost 6 hours more than 365 days to complete one full revolution about the Sun. Because the Earth takes almost 6 hours less than 365 days to complete one full revolution about the Sun. To make up for an inadequacy in our modern calendar. To make up for the slowing of the Earth’s rotation.

“Right Ascension” measures east-west position on the celestial sphere RA is measured in hours, minutes, and seconds “Declination” measures north-south position on the celestial sphere Dec is measured in degrees, arcminutes, and arcseconds

Astro-Cash Cab! Jessica Chiles Merlin Ginther Erin Henderson Daniel Lovell Nick Kirpes

1) Which statement is false? A) A solar day is always longer than a sidereal day. B) The length of a sidereal day varies over the course of a year. C) A synodic month is longer than a sidereal month. D) Only inner planets have two kinds of conjunctions.

2) Which calendar do we use today? Egyptian calendar Julian calendar Gregorian calendar Mayan calendar

3) True or False? When Venus is at Greatest Eastern Elongation, it is in the eastern sky at sunrise.

4) What technological advancement led to the development of time zones?