By Bill Pellerin
Houston Astronomical Society
GuideStar Editor

Just after midnight (Central Daylight Time) on June 21, summer began officially. You would be forgiven (in some parts of the country) if you thought that summer was here prior to that date, with some especially hot days. June 21 was the summer solstice, the day on which the Sun is the highest in the sky and the longest sunlit day in the year. At the solstice the Sun is at its northernmost excursion.

I live in Houston, Tx, where the latitude is 29.8 degrees north. Does the Sun ever get directly overhead here? No, it doesn’t. That is, the Sun does not reach the zenith. You’re probably aware of two lines on the world map called the ‘Tropic of Cancer’ and the ‘Tropic of Capricorn’. These two lines represent the northernmost and southernmost positions of the Sun at summer solstice and winter solstice. Does the Sun get to the zenith at any point in the continental United States. Again the answer is no.

The northernmost position of the Sun this year is 23 degrees, 26 minutes. So, in my hometown, the Sun was about 6 degrees south of the zenith on June 21 as it crossed the meridian, and close to M35 at the foot of Gemini. How about other locations? Brownsville, Tx has a latitude of 25.9 degrees; still too far north. Even the southernmost location in the continental United States Ballast Key, Florida at 24.5 degrees north is just about one degree too far north. The southernmost point of the entire United States is at Ka Lae, Hawaii at 18.9 degrees north, so clearly the Sun will reach the zenith at this location and at other places on the big island just north of this site.

The good news about the summer solstice, of course, is that the amount of darkness per day increases from now until December 21, so we’ll have a bit more dark time every day. How much more dark time? Well, the velocity at which the Sun moves south in the sky varies over that time, but, on average, Houston gets about 1.6 minutes more of darkness every day between now and December 21. This is based on astronomical twilight, which is when the sky gets as dark as it’s going to get (not considering moon phase). On June 21, we got just over 6 hours of darkness and on December 21 we’ll get almost 11 hours of darkness. We’ll have two and a half hours extra of darkness in the evening and two and a half extra hours of darkness in the morning. I’m not taking into account the changes in clock time associated with daylight saving time.

If you live north of Houston, the change in dark-time between summer and winter is even higher.

The north to south (summer to winter) movement of the Sun from our viewpoint is a manifestation of the tilt of the axis of Earth. On the first day of summer, the southward velocity of the Sun is near zero. At the autumnal equinox on or near September 21, the southward velocity is a maximum, at the winter solstice the north-south velocity of the Sun is near 0 and it’s a maximum south to north velocity at the vernal equinox on or near March 21. I calculated the change in hours of darkness (based on USNO data) over the year and plotted the result. As you might expect the result looks like a familiar sine wave. There would be some variance from a sine curve because the orbit of Earth around the Sun is slightly elliptical, not circular.

The US Naval Observatory has a web site that shows hours of darkness or daylight for various cities in the U.S. and around the world. The numbers in their calculations don’t match mine because I’m using astronomical twilight time and they’re using (essentially) sunset time. (Search for “USNO hours of darkness” and you’ll find the web site.)

By Bill Pellerin
Houston Astronomical Society
GuideStar Editor

If you have a world globe in your house it may include a strange figure 8 pattern, called the Analemma, with some dates on it. Without going into great detail, this shows the north/south position of the Sun in the sky for the year. Because the orbit of Earth around the Sun is elliptical, the Sun is not always on the meridian at mid-day. In fact, the Sun is only on the meridian at mid day on the two equinox days and on the two solstice days. To keep ourselves going to meetings our time clocks show the average length of the day as 24 hours. Close enough. On some days the Sun is east of the meridian at noon and on some days it’s west of the meridian at noon (always ‘standard’ time). The Analemma shows you how this goes.

So for the next 6 months, revel in the notion that you’re getting more dark sky every day. Take advantage of it.

ALCon 2013 Flyer.

ALCon 2013 Flyer with white background for printing

Library lending telescope flyer

Registration form

By Bill Pellerin, Houston Astronomical Society

GuideStar Editor

If you’ve been outside on a dark night, looking up you’ve probably seen a number of satellites move across the sky. I see them in my eyepiece from time to time too; it’s a fairly common occurrence. They zip by in a hurry and I haven’t made an effort to figure out which one I saw. If you’re an imager you likely don’t want a satellite to be recorded on your camera. It happens, though, and there are image processing techniques to fix those problems.

But, what if you want to see a satellite? If you want to see a satellite, such as the International Space Station, go by it’s not difficult. You need to know where in the sky to look and when. The simplest way to find out, go to www.heavens-above.com. Once there you can register (no cost), and save your location, or you can simply pick your location, pick the ISS and get a report of visible passes in the next 10 days. The visible passes show where the satellite will first show up in the sky (altitude and azimuth), how high it will get in the sky, and when and where it will be when it disappears from view. You can look at a sky map for the event, too.

Satellites are visible to us in the night sky because they can be in sunlight while the observer is in darkness. If the satellite is 300 miles above Earth it will be sunlit later in the evening than we will, with our eyes just a few feet above Earth. This means, that for many of the satellites you will only be able to see them for the few hours after sunset or the few hours before sunrise. Think about this. If you are outdoors and it’s just getting dark after sunset the Sun is below your western horizon. A satellite moving (conveniently) west to east would be illuminated by the Sun and reflect some of that sunlight to your eyes. As the satellite moves east it likely will move into the shadow of Earth before it gets to the horizon.

There are satellites at various altitudes above Earth. Many of them are LEO (Low Earth Orbiting) satellites. The Space Station is at about 230 miles above Earth and would be considered a LEO satellite. If you use the common size for Earth (8000 mile diameter) and scale that down to the size of a desktop globe, say 12”, the Space Station is about 1/3” above the surface of the planet.

That said, a LEO satellite, being closer to an Earth-based observer will (all things being equal) be considerably brighter than another satellite at very high altitudes. How bright any satellite appears to you will depend on the size of the satellite and the reflectivity of the satellite as well.

How about geosynchronous satellites? Those are the ones whose orbital period is equal to the (sidereal) period of rotation of Earth. That is, they must be high enough so that the period of one orbit equals 23 hours, 56 minutes, and 4 seconds. It turns out that if you do the math, the satellite must be placed in orbit just over 22,000 miles above Earth to be geosynchronous. This is commonly done for satellite radio satellites and communication satellites of various types. For our 12” Earth globe, these satellites would be roughly three Earth diameters away.

Can you see these? Yes, you can but because these are dime seeing these will require a telescope and a precise understanding of where these objects are on the sky. Since they’re moving at a sidereal rate, a telescope that tracks the stars will also track these satellites.

Commonly, amateur observers who aren’t primarily satellite observers like to see the Space Station, the Hubble Space Telescope, and missions to and from the Space Station. These are easy to do on dark nights without any optical aid. I recall seeing the Space Station go over with the Space Shuttle trailing along behind it (some years ago, of course). The Shuttle was leaving the Station on its way back home.

The other very cool thing to see is an Iridum flare. These sound more exotic than they are. These communication satellites have large solar panels, and when the orientation is just right there’s a very bright reflection of sunlight from the solar panels to your observing position. I’ve seen a bunch of these and it’s definitely worth your time to step outside and see one. The heavens-above web site can predict when one will be available for you to see.

There is also software you can install on your personal computer or on your mobile device. Some astronomy software has satellite tracking capabilities built in.  I use TheSky for my astronomy work, and it will track satellites across the sky. I have GoSatWatch on my iPhone; it’s a paid application. GoSatWatch allows me to pick the satellites that I want to view (ISS, HST, etc.) and it will alert me when a visible pass is coming. It also estimates the magnitude (brightness) of the satellite, and shows an all-sky view of the pass. Once the pass starts, the software shows the position of the satellite in the sky in real-time. Many of these satellites are quite bright so you can see them from an urban observing location.

If you want to try to see a satellite through your telescope (or take its picture) you need some special software to drive your telescope at the satellite rate. Satellites move across the sky quickly. Join the Yahoo Group ‘satellitetracker’ to learn about doing this. (Go to groups.yahoo.com.) I’ve seen some really nice images of the Space Station taken by amateurs. I also have seen an image if the ISS transiting the Sun.

Want to talk to an astronaut aboard the ISS or communicate through an earth-orbiting satellite? You can do this by amateur (ham) radio. To get your license go to www.arrl.org/get-on-the-air and find out how. To find out more about amateur radio satellites go to www.amsat.org. To communicate to the ISS or through a satellite you’ll need to point your antenna directly toward the satellite. If you have a telescope mount that can be driven at the satellite rate it may be possible to attach the antenna to the telescope mount and let the mount do the real-time pointing for you. There are also alt/az antenna rotators you can use to point your antenna at satellites.

The Astronomical League has a program called Earth Orbiting Satellite Observing. It is created by the Colorado Springs Astronomical Society and it will introduce you to the process of observing satellites. Check this out on the Astronomical League web site.