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Latest sunsets of year follow solstice

Sunset at White Rock Lake near Dallas by Lucy Bee.

Tonight for June 27, 2015

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 Northern Hemisphere’s longest (or Southern Hemisphere’s shortest) day of the year falls about one week earlier, on the June solstice.

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 21 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 06s

Source: timeanddate.com

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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.

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, 2015.

What else can you see on the night of June 27? Try looking for the moon and noticing the bright points of light near it? One is the star Antares, brightest light in the constellation Scorpius the Scorpion. Another is the planet Saturn. Saturn will be even closer to the moon on June 28.

Watch for the moon to sweep past Saturn on June 27, 28 and 29, 2015.

Watch for the moon to sweep past Saturn on June 27, 28 and 29, 2015.

Also, the gorgeous pairing of the sky’s brightest and second-brightest planets – Venus and Jupiter, respectively – can be viewed in the west-northwest sky at dusk. At mid-northern latitudes, the celestial couple stay out for about 2.5 hours after sunset.

Best photos: Venus and Jupiter, west after sunset

The first two

The first two “stars” to pop out into the western sky after sunset are the planets Venus and Jupiter.

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.

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