For people living around 40o north latitude, the latest sunsets of the year happen in late June. And in the Southern Hemisphere, at 40o south latitude, it’s the year’s latest sunrises that happen around this time of year. That’s in spite of the fact that the longest (or shortest) day of the year falls about one week earlier, on the June solstice.
Although the latest sunset always comes on or near June 27 at mid-northern latitudes every year, the sky scene in the northwestern dusk is special to June 27, 2013. As dusk ebbs into darkness at mid-northern latitudes, look for the Venus, the sky’s brightest planet, to line up with the stars Castor and Pollux over the northwest horizon. On this date next year, in 2014, it’ll be the planet Jupiter that joins up with stars Castor and Pollux over the northwest horizon.
The year’s latest sunset always comes after the summer solstice, even though the exact date of the latest sunset depends on your latitude. Farther north – at Seattle – the latest sunset happens a few days before June 27. Farther south at Mexico City, the latest sunset won’t happen until early July.
The latest sunsets come after the summer solstice because the day is more than 24 hours long at this time of the year. In June and July, the day (as measured by successive returns of the midday sun) is nearly 1/4 minute longer than 24 hours. Hence, the midday sun (solar noon) comes later in late June by the clock than it does on the June solstice. Therefore, the sunrise and sunset times also come later by the clock, as the table below helps to explain.
For Denver, Colorado
|Date||Sunrise||Midday (Solar Noon)||Sunset||Daylight Hours|
|June 20||5:32 a.m.||1:02 p.m.||8:31 p.m.||14h 59m 18s|
|June 27||5:34 a.m.||1:03 p.m.||8:32 p.m.||14h 58m 01s|
If the Earth’s axis stood upright as our world circled the sun, and if, in addition, the Earth stayed the same distance from the sun all year long, then clock time and sun time would always agree. However, the Earth’s axis is titled 23.5o out of vertical, and our distance from the sun varies by about 5 million kilometers (3 million miles) throughout the year. At and around the equinoxes, solar days are shorter than 24 hours, yet at the solstices, solar days are longer than 24 hours.
Bottom line: Why don’t the latest sunsets come on the longest day (the solstice)? In a nutshell, it’s a discrepancy between the sun and the clock. Thus, for mid-northern latitudes, the latest sunsets always come in late June.