Last night was hot – the first throw-off-the-blanket night of the year!
An unlikely reason was that the Moon was bright enough to penetrate through our window blinds. It is full today, June 14.
More relevant is that June 14 is the height of summer, in one sense: sunrise was earliest (about 4:30 by standard time) for places on Earth at latitude 40° north.
Why does earliest sunrise come a week before the longest day, the June 21 solstice, and why does it vary with your location? This is explained rather fully in our page about “Latest and earliest sunrise and sunset.”
And, thirdly and absurdly, we could fancy that the little planet Mercury is striving to reflect a few more photons of sunlight to us than usual: growing brighter, and reaching, on June 16, its greatest western elongation, that is, its farthest out into the morning sky.
See the end note about enlarging illustrations.
In sky scenes like this, most of the features shown will not be visible in the twilight sky, but they serve to give a sense of which way we are facing in space. Arrows through the moving bodies, including the Sun, show their motion from 2 days before to 2 days later, in relation to the starry background. The symbols for the planets are sized for brightness like the stars, except that the disk of Venus is shown, swollen 150 times in size.
Mercury is 23° from the Sun and shines at magnitude 0.6, like some of the brightest stars. But it is, at the time and location of our picture, only about 3° above the horizon. For it is in a part of its orbit that lies south of the ecliptic, putting it at a depressed angle from the Sun.
This graph from page 104 of Astronomical Calendar 2022 show how this June apparition of Mercury compares with the others in the year.
The gray and blue areas are evening and morning excursions. The top figures are the maximum elongations, reached at the top dates shown beneath. Curves show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40° north (thick line) and 35° south (thin), with maxima reached at the parenthesized dates below (40° north bold).
Though Mercury goes out to quite a large elongation, the height it attains above the north-hemisphere sunrise horizon is middling-low. For South Africa, where the horizon slopes the opposite way, the Mercury apparition is much more conspicuous.
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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image” or “Open image in new tab”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.
Sometimes I make improvements or corrections to a post after publishing it. If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version. Or you can click ‘Refresh’ to get the latest version.
This weblog maintains its right to be about astronomy or anything under the sun.
Not so hot in Sydney at the moment, with the coldest start to winter in a very long time.
And this afternoon I’ve just flown to Wellington in NZ, where it’s windy as usual and none too warm.