Sun spotting in safety at solar maximum
With Scotland’s nights still awash with twilight, many people focus on the Sun during July. There are dangers involved, though, and I don’t just mean sunburn. Specifically, we must never look at the Sun directly through binoculars or any telescope. To do so invites serious eye damage. Instead, project the Sun’s image onto a white card held away from the eyepiece or obtain an approved solar filter to fit over the objective (rather than the eyepiece) end of your instrument.
The most obvious features on the solar disk are sunspots, cooler areas that are shaped by magnetic activity and last for a few hours to several weeks. Because the Sun rotates every 27 days with respect to the Earth, spots take two weeks to cross the Sun’s face, provided they survive as long.
Sunspot numbers ebb and flow in a solar cycle of about 11 years, although the actual period varies from about 9 to 14 years. The last peak in the Sun’s activity occurred in 2000 and, following an unusually prolonged minimum between 2007 and 2010 when very few spots were seen, we are back near solar maximum though at a lower level than in 2000. This also means that solar flares, and the auroral displays that they can produce, are also more frequent even if they are hard to see given our summer twilight
As I warned last time, though, silvery or bluish noctilucent clouds are sometimes visible low down in the northern quarter of the sky and Scotland enjoyed a nice display on the night of 19-20 June. They are formed by ice crystals near 82km and more can be expected until mid-August or so.
The Sun tracks 5° southwards during July and from the 12th onwards Edinburgh enjoys at least a few minutes of official nautical darkness around the middle of the night. We need to wait a few days more for the bright Moon to leave the scene, but when it does the fainter stars should once again be visible.
If light pollution is minimal, the Milky Way may be seen arching high across the east at our star map times. Marking the central plane of our galaxy, with the greater density of distant stars, it stretches from Capella in Auriga in the north through the “W” of Cassiopeia in the north-east before flowing by Deneb in Cygnus in the east and downwards towards Sagittarius near the southern horizon. Where it passes through the Summer Triangle formed by Deneb, Vega and Altair it is split into two by obscuring interstellar dust, the Cygnus Rift.
The red star Chi Cygni, 2.5° or five Moon-widths south-west of Eta in the neck of Cygnus, pulsates every 13 months or so between a naked eye object of the fifth magnitude and a dim telescopic one near magnitude 13. It reached an unusually bright peak of better than magnitude four last year and should be near maximum again about now, though recent observations suggest it may not even hit magnitude six this time.
The Earth is 152,114,000 km from the Sun, and at its farthest for the year, on the 4th. Sunrise/sunset times for Edinburgh change from 04:31/22:01 BST on the 1st to 05:15/21:22 on the 31st when nautical darkness lasts for almost four hours around the middle of the night. The Moon is at first quarter on the 5th, full on the 12th, at last quarter on the 19th and new on the 26th.
Jupiter is barely 6° above the west-north-western horizon at sunset on the 1st and is unlikely to be visible as it heads for conjunction on the Sun’s far side on 24th.
Mars, to the right of Spica in Virgo and low down in the south-west at nightfall, sinks to set in the west-south-west at our map times. Fading from magnitude 0.0 to 0.4 this month, it tracks to the left to pass 1.3° above Spica on the 14th – the final and closest of three conjunctions between them this year.
The young Moon lies below Regulus in Leo low in the west on the evening of the 1st and close to Mars on the 5th. The 7th finds it close to Saturn and even closer to the double star Zubenelgenubi in Libra, the three making for a superb sight through binoculars. Saturn dims only slightly from magnitude 0.4 to 0.5 and hardly moves against the stars, appearing telescopically as an 18 arcseconds disk with rings 40 arcseconds wide.
A brilliant morning star at magnitude -3.9, Venus rises at about 03:00 BST and stands 12° to 14° high in the east-north-east at sunrise. As it tracks eastwards through Taurus, use it as a pointer to Mercury which is less than 8° below and left of Venus from the 10th to the 23rd as it brightens from magnitude 0.8 to -0.8. Set your alarm to catch Venus 8° to the left of the 7% illuminated waning earthlit Moon before dawn on the 24th.
While many stars are larger than our Sun, including the vast majority of stars visible to the unaided eye, there are billions that are smaller. Indeed, red dwarf stars, from about half the Sun’s mass to 1/13th as massive, are thought to make up 75% of the more than 100 billion stars in our galaxy. The smallest known star, and probably close to the smallest star possible, is a red dwarf in the constellation Lepus, just south of Orion. Smaller than Jupiter, but more massive, it has surface temperature of 1,800C and a luminosity of 1/8,000th of our Sun so that we need a large telescope just to see it even though it is only 40 light years away.