Caoimhín's Content

Today we are going to take a look at the sky for later in the month of July, starting with the stars and the constellations that we’re seeing before moving on to some planets at the end. We are finally getting away from the summer solstice, so the days are getting shorter. That means we’re going to have longer nights and things are starting to get darker. We are now, at least here in Ireland, moving away from the time of year where we have twilight all night long. We’re finally getting back to nights where we actually have true nighttime. Soon we will be past astronomical twilight for at least some portion of the middle of the night.

Just after the Sun goes down, Venus is visible setting, with the Moon around mid-July. Venus is still above the horizon, but it is very low in the sky by the time it appears. The Moon is coming back to full, we will of course have our Full Moon close to the end of the month, and it will be very low in the sky. The Sun is still getting very high in the sky during the day, the ecliptic is high in the sky during the day, and so the ecliptic will be low in the sky at night. That can even be seen with the constellations on the ecliptic, like Scorpius and Sagittarius, just along the bottom of the horizon after sunset. Those constellations that are on the ecliptic, the zodiacal constellations, they are much easier to see during the winter because they’re higher in the sky. If you remember the constellation Taurus, for example, or Gemini, that we were seeing closer to mid winter, they were really high in the sky, because that’s where we were seeing the ecliptic at nighttime.

Now instead, we have the summer triangle higher in the sky, and constellations like Boötes, who’s brightest star Arcturus is pointed at by the Plough’s handle. Between the Triangle and Boötes we’ve got Corona Borealis and Hercules. Hercules is always a tricky shape to spot because it is a faint trapezoid, it might take a binoculars to see the full trapezoidal shape of Hercules, and there are bits branching off which are parts of Hercules as well. Iit is tough to see these details when we’re looking at the sky firstly in a city and secondly with some light from the Sun still in the sky. We need to push later to get through to actual darkness, or very nearly actual darkness at least, there is still a glow above the northern horizon even as we come up to our local midnight. The Sun is directly behind the north, instead of our local noon, where the Sun is directly in the south. That extra bit of light, it’s still there even though we are past the middle of the month now, but as we move forward we can see that light receding. The the light of the Full Moon will come up, brightening the sky. Once the Moon’s gone, we’re into August and there’s no more light of the Sun. We are getting back to the time where we will have true nights again but we’re not quite all the way there until the end of the month, there is still a little bit left.

Looking into the north we can of course see the Plough or the Big Dipper, it’s getting down to that low position that it’s going to have in autumn as the Sun goes down. We’ve also got Cassiopeia, that kind of W shape up to the other side of the north star, it’ll be getting higher in the sky as we move into the future. Moving into August and the Plough or the Big Dipper is practically at the lowest point of the sky in the north by the time we’re through to our local physical midnight. Soon after we’ll be leaving summertime. In wintertime we’ll be using UTC +00, the actual Universal Standard Time, we use that during the winter, but during the summer we’re an hour ahead due to daylight savings time.

As we’ve now moved a little bit later in the month, Saturn is already up as we come past midnight, at 12:30 it’s just risen. It’s very low in the sky and that means it’s going to be extincted by the atmosphere. As we approach morning time it’s fairly high in the sky, because we’re seeing it closer to sunrise we’re seeing the angle of the ecliptic a little bit higher. The angle of the ecliptic is a little bit lower as we’re coming into sunset. Where we see the ecliptic is due to the angle of the Earth and the way the Earth is tilted, but where we’re seeing these planets also depends on their positions in sky. Where we see the ecliptic is due, a lot of the time, due to the tilt of the Earth and the way the Earth is tilted but where we see the planets isn’t just to do with where we see the ecliptic, the planets are slightly inclined in their orbits so they can appear above or below the ecliptic. Regardless, by sunrise we have Saturn and Mars.

We will take a closer look at Saturn simply because it’s been a while since we’ve taken a closer look at Saturn. Saturn’s rings are now looking pretty nice, as are all of these moons as well. Saturn’s moons of course, are fantastic, Titan in particular is visible through reasonably small telescopes. Titan’s especially visible at its furthest from Saturn, going back to its furthest from Saturn on either side. There is some twinkling or extinction when we look into Saturn a week or so ago, that’s because Saturn was lower in the horizon so there’s more atmosphere involved. As we move forward, it’s higher in the sky, so we’re getting less of that strange twinkling effect. However, the reason I wanted to take a look at Saturn is because Saturn’s rings have been quite thin recently. As we move back in time the rings are looking thinner, but as we’re moving forward the rings are looking a little bit thicker. This is good, Saturn has its own tilt and the way we see Saturn and how we see its rings does change over time, but it’s nice to be able to see Saturn’s rings looking wide because when Saturn’s rings are almost edge on, they’re very difficult to see.

While we’re looking at the planets, before we take a closer look at Mars, we’ll quickly look around Saturn’s region of the sky, to spot Neptune out in that direction not too far from Saturn. Vesta, one of the larger asteroids, is visible out in the direction of Saturn as well. Neptune may be harder to see than Vesta, even though Neptune is much larger it’s much further from us. Vesta is quite close, it’s just beyond Mars. Closer to Mars is Uranus, just barely visible as we zoom in. Uranus is visible through binoculars and small telescopes, but definitely in a city sky it’s not quite visible to the naked eye. We’re also getting to see the Pleiades fairly well and the Hyades of Taurus fairly well there, so we are getting those back in the morning.

Taking a closer look at Mars, it looks like Mars is going between northern winter and southern summer. As we move forward, the northern hemisphere tilts towards us and the ice cap is getting smaller even as Mars gets closer to us. I wanted to double check that Stellarium does this accurately, and it does. At the peak of northern summer, the northern ice cap has more or less disappeared while the southern ice cap has grown. This ice cap is mostly dry ice, it’s frozen carbon dioxide. Carbon dioxide makes up a lot of Mars’s very thin atmosphere. As the planet’s tilt continues to shift, at Mars’s equinox the planet looks upright or level. Moving into southern summer, the southern ice cap gets smaller. Now we’ll have to zoom in because Mars is further from us, but the southern ice cap is now looking much smaller. Mars, just like the Earth, has seasons, a tilt, and even ice caps. The ice cap on one side gets bigger and the ice cap on the other side gets smaller, but rather than water melting and flowing into the ocean, what’s happening here is carbon dioxide, or dry ice, is subliming, which means turning into a gas, and entering the atmosphere on one side of the planet while the other side of the planet gets colder and the carbon dioxide freezes back down onto the ground. At the moment, Mars is coming up to its equinox, in between southern summer and northern winter. Also visible are the two moons, Phobos and Deimos. It’s always nice to take a closer look at the planets and Mars in particular is an interesting one, it’s a close neighbor of ours.

While we’re looking out here, near Mars is a comet, 78P/Gehrels, discovered in 1973. It’s pretty faint, it’s only 13th magnitude and I’m not seeing much of a tail. Given that it’s twice as far from the Sun as we are, I wouldn’t expect to but that is another comet that’s moving its way in our direction. Just as we finish up, we’ll come back to sunset, now that we’re looking a little bit closer to August. We know that Sagittarius is in the south. The square of Pegasus is just above Saturn, so between them must be Aquarius and Capricornus. Out in this direction is another comet, 10P/Tempel. It is 6th magnitude, much brighter than the 13th magnitude. Again, we’re looking a week or so into the future, but that would be just on the edge of visibility in a dark sky. That’s a comet that we may actually be able to see, certainly with binoculars it’s a very achievable target. It is, however, quite low in the sky, so that might make things difficult.

That’s just a quick look at some of the things that are coming up as we get to the end of the month, before we get into the very exciting month of August with a solar eclipse and the Pleiades meteor shower and other interesting things. I intended to concentrate mostly on the constellations, but it’s worth taking a closer look at two planets, they are interesting things that we’re getting to see in the morning sky. Venus, we’re just about catching a glimpse of in the evening sky, quite low.

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