Star Points for January, 1998; by Curtis Roelle The Sun is Brighter During Winter You might find comfort knowing that today, Sunday January 4, Earth is at its closest point to the Sun for the year, or "perihelion". This is five million kilometers (3.1 million miles) closer than we were at our farthest point, "aphelion ", on July 4th according to editions of Guy Ottewell's "Astronomical Calendar" for 1997 and 1998. The Sun is at its brightest and the Earth receives more radiation when the northern hemisphere is experiencing winter. That's good news for us -- every extra bit of sunshine helps. But shouldn't we pity the other half? Those poor folks down under Earth's equatorial belt who are now experiencing summer must be sweltering due to the additional heat caused by the Sun's proximity, correct? Surprisingly the answer is no. As Ottewell explains in "The Astronomical Companion" (printed 1995) it is the other way around: Summers are hotter in the northern hemisphere when the Sun is farther away, than summers in the southern hemisphere when it's closer. This is because most of the Earth's oceans are in the southern hemisphere and water absorbs and radiates heat slower than land. The reason the Earth wanders closer to and farther from the Sun during the year is because its orbit is not a perfect circle. Rather the shape of the orbit is an ellipse. Surprisingly, this has no bearing on our seasons or calendars. Winter in the northern hemisphere began on December 21. That was the day of "Winter Solstice", when the Sun is overhead at the Tropic of Capricorn in the southern hemisphere (Latitude 23.5 south). In the northern hemisphere on that day we experienced the longest night of the year and hence the shortest day. In Westminster the Sun didn't rise for 14 hours and 38 minutes after setting. It seems logical that we experienced the earliest sunset and the latest sunrise on that day. However in astronomy things are often not what they seem. In fact our earliest sunset occurred two weeks earlier, on December 7 at 4:44 p.m. EST. The latest sunrise occurs tomorrow on Monday, January 5 at 7:29 a.m. EST. What causes this apparent discrepancy? The reason that the earliest sunset and latest sunrise do not occur on the day of the Winter Solstice is the same reason that the distance between Earth and Sun changes during the year: The non- circular, or "eccentric", shape of the Earth's orbit. The 17th century astronomer Johannes Kepler discovered that planets speed up when they approach perihelion and slow down near aphelion. Our clocks and calendars are based on the "mean" (or average) apparent motion of the Sun in our sky. When our changing distance from the Sun causes Earth to speed up or slow down the Sun appears to run "fast" or "slow" relative to its average apparent daily motion. According to an article which appeared in the December issue of "Astronomy" magazine, the Sun runs "fast" before the solstice and "slow" after. Another reason for the occurrence of earliest sunset and latest sunrise on different days is the tilt of the Earth's rotational axis with respect to the plane of its orbit around the Sun. For history buffs it might interest you to know that according to the obsolete Julian calendar Wednesday, January 14 is New Years Day. When the contemporary Gregorian calendar was first adopted by the Catholic church in 1582 ten days had to be eliminated from that year's calendar to make up for the fact that the Julian calendar had been "gaining". By the time protestant England switched over to the new system an additional 11th day needed to be purged. This two calendars have now diverged by 13 days. According to Ottewell, a 14 day difference will be reached in the opening days of the 22nd century on January 15, 2101. Whether you have adopted the Gregorian calendar or are sticking to the Julian calendar, here's wishing you a Happy New Year!