# Geometry and The Moon

Please do not run away. We are about to use adult language here. For example we will be using the word trigonometry. Still here? Good.  Here is a very pedestrian looking lunar eclipse photo taken with a 280mm lens*, cropped.

Very Ordinary Photo of the Lunar Eclipse with the planet Uranus in the lower left.

This past lunar eclipse several of us put our heads together to try to come up with a more creative photo than the one above. We had a trigonometry problem, however. On the West Coast the last moment of totality occurred at 4:24 AM PDT. We were brave enough to be out at any time of night – even if it meant extreme sleepiness in our day jobs but our problem was that the lowest the moon would be in the sky at the last bit of totality was 32.6 degrees above the horizon. We determined that angle using Stellarium, by the way. Unfortunately there is pretty much nowhere to go to get a nice large moon near an interesting object when the moon is almost 33 degrees high.

Wait: Why do we want the moon and the object to be similarly sized? Here is why… we want the moon to be noticeable like the Fantasy version below, not merely “present” like the real photo on the right. Even bigger would be better, right!?

Notice above right (Reality) and below how tiny the moon is compared to the building in the foreground?  Indeed, if you see a photo taken from anywhere on the West Coast where the eclipsed moon is significantly lower in the sky or larger than shown against foreground, you know it has been “photoshopped“.

Plan C: San Jose City Hall Eclipse Sequence

In short, it is nigh impossible to get the large moon effect with an altitude (angle) of 32 degrees here is why:

Calculating the Angles

Just how far away do we need to be in order to get the moon the same size as an object of interest:

114.6 x object size

In other words, an object that is one foot tall, requires us to stand 114.6 feet away to make the 1/2 a degree angular size of the moon the same angular size as that 1 foot tall object.  The number “114.6” is from this calculation:

1 / TAN (0.5 degrees)

Yeah, that is trigonometry. Using still more trigonometry it is possible to calculate how high above the horizon a 9 inch tall object has to be so that it is “moon sized”.  We did that for you in the “Calculating the Angles” diagram above. Once you calculate the distance from the camera of 85.9, you can multiply that by the sine of the angle to calculate a height of about 46 feet! Here is the trigonometry:

Height = 85.9′ * SIN (32 deg)

You can go one step farther and calculate the distance from the object with ‘distance = 85.9 * COS(32 deg)’.

Of course after all that calculating you will still need to find a location, have contingency plans for weather and so on. At StarCircleAcademy we have built some tools and put together materials to help in all these endeavors.  We teach these things in our NP111 Catching the Moon Webinar.

# The Road To The Temple

Below is where we ended up. This image is from our friend and co-conspirator Andy Morris.

Lunar Eclipse over Temple by Andy Morris of PhotoshopScaresMe

Four of us plotted and schemed to get an interesting shot. Above is Andy Morris’ result.  Click the image and you can read a great article about how he created the shot using Photoshop Skills at his site: PhotoshopScaresMe.com. In fact, it’s a great article which we strongly encourage you to read. You’ll learn how he composited the images together in Photoshop as layers.

### The Long Conversation to Pick a Location

Andy has more details including how alcohol played a part in the process. Mostly I, Steven, was the wet blanket explaining why the geometry was all wrong.

• The Stanford (Hoover) Tower looks like it is shrouded in trees from the needed angle
• Bank of Italy (formerly BofA) in SJC doesn’t work
• The main problem with the wind turbines is that the angle to the top of them is something around 12 degrees above the horizon which is 40 moon diameters below the eclipse.
• Here is why the GG Bridge doesn’t work…
• This seems to be the best solution I could find: the Coit Tower…
• Darn. It would appear the coast is out. Forecast calls for Fog from SF to HMB
• This might make an interesting foreground (see below)… Somebody want to check if they will mind us being on their property in the wee hours?

*Ok, we lied, it was actually a 70-200mm lens with a 1.4 TC on a full frame camera, but the net is the same: 280 effective mm focal length.

Where did you go and what did you get in your planning efforts?  Post a comment and link below… we’d love to see what you came up with!

# Planispheres (Star Maps): Paper or Electronic?

Published May 29, 2014
Last Updated April 18, 2016

A topic that comes up a lot is discussion about what makes a good astronomy helper application. Whenever we suggest purchasing a paper Planisphere our critics remind us that they are not necessary because “there is a great app” to do that.

We take exception to the “there is an app for that” assertion… but perhaps not for the obvious reason. In fact we DO use several apps for forecasting and navigating the night sky. But ultimately we find the good old fashioned planisphere to be the most effective for most of what we want to do. We’ll make the case for a paper (or plastic) planisphere in a moment.

## Why Do We Want Something Besides our Eyes?

Let’s start with determining why we want something to help us with our night sky navigation. Some scenarios to consider include:

1. We are a beginner and we really don’t know Canis Major from Major Appliances.
2. We have some familiarity with some of the constellations, but we want to learn more.
3. We want to take a shot with a particular sky object behind a particular landmark.
4. Even though we know the night sky pretty well, we still need to be able to find faint objects, or find objects in less than dark skies – the Milky Way, for example is difficult to see unless conditions are good and the sky is dark.
5. We are going to go to an unfamiliar place with a latitude that is very different from where we normally gaze at the night sky.
6. We want to know where to look to observe a particular phenomenon like the Geminid Meteor shower.

## Can’t I Do that with an App?

It might seem that and android, iPad or iPhone app is the best tool since you can take it with you.  And that MIGHT be right except for the following significant problems:

1. Unless you keep the app brightness really low or use it in a “dark sky mode” (usually dim red), you’ll damage your night vision making it difficult or impossible to see dimmer objects in the night sky.
2. If you’re trying to find the Milky Way (the dense part in Sagittarius) but you try to use the app during a period when the Milky Way is not visible. No matter what time of night you enter, you won’t see the Milky Way (e.g. November through January).
3. The representation on the app is often NOTHING like what it may look to your eye in your location. Every app suffers from this problem in one way or another. Some apps make the Milky Way in Canis Major appear to be incredulous – actually its very sedate there.
4. You want an idea when it will be BEST to get the Milky Way aligned over your target. But on an App you will need to determine the time manually.
5. If you mistakenly trust the app to tell you where it’s pointing you may be surprised how wrong it can be. Due to iPhone, Android, and iPad hardware limitations, a handheld app could be anywhere from close enough to off by 180 degrees!  It will be even worse if for some reason your App is configured for the wrong timezone, or the wrong GPS location.
True story 1: I happened on a family in Yosemite, California and the dad had out his iPad pointing out to his children: “see … there is Orion”.. and over there…” but he was from Alabama and his iPad was off by 3 hours – and his compass wasn’t calibrated either so he ended up almost 180 degrees off.
6. Dead battery. If you have to choose between enough battery to make an emergency call or figuring out your night sky… well, we recommend saving the battery.
7. Most apps show only a fractional portion of the sky which may confuse anyone who is not already familiar with the sky.

While we freely admit that we like and use the following applications, we prefer a paper/plastic Planisphere.

• Stellarium – FREE runs on Mac, PC and Linux.  We like it because it has excellent sky condition simulations that help give a realistic view of the night sky.  It won’t show you dim stars under bright moonlight unless you ask it to. It can also track comets and satellites. What we don’t like is that it is fidgety to configure.
• StarMap by Fredd software for the iPhone/iPad. We like this one because it’s quite complete. It is well organized to show you, for example, what meteor showers are visible, what “dimmer” objects you can find, and has a simple interface for adjusting the sky brightness or the time of day. What we don’t like: we like to call the constellations by their scientific (and we believe) more common names.  Herdsman?  That’s Bootes. Big Dog? That’s Canis Major, thank you.  Note there are TWO versions of this App, unless you’re a serious astronomer, the less expensive one will work.
• GoSkyWatch – admittedly we like it because we got it as a free app through Starbucks app of the week but we think its worth the price anyway! We like that it’s pretty versatile, when you point it at the sky it gives the altitude and azimuth (elevation angle and compass direction) which can come in quite handy – even though as we’ve already noted the compass direction is probably wrong! Zero in on an object and it will give you and idea what it looks like. We like that it’s Milky Way representation – while overly bright is pretty close to what it looks like. You won’t confuse Canis Major with Sagittarius, for example. It also includes a great assortment of dim objects and shows constellations with “good names” not just the “common name”. It also has a night mode to conserve your night vision.  It doesn’t have meteor showers or satellites, however.

## What We Don’t Like

We’re not fond of anything we haven’t listed. Not that there aren’t better apps, but every one we’ve tried falls short in some way. Take for example, SkySafari.

SkySafari for example, is mostly a disappointment. Not only are there 12 different versions for iOS that range in price from \$1 to \$40, but the app doesn’t do a good job simulating the night sky, prefers to show useless images of the mythological constellations (which fortunately can be turned off) and shows a garish orange Milky Way which might be exciting to look at except that it will never look like what the app reveals.  SkySafari also doesn’t adjust for the effects of twilight or moonlight.

SkySafari does have some nice information about each object in its database, but the database is not searchable.  If you’re interested, for example in M101 you’ll have to scroll all the way to the bottom of the Messier Catalog page.  If you want to catch a glimpse of the ISS (Zarya/Space Station) you’ll have to slog through the Satellites page.

## Why We Like the Planisphere

In this day and age it’s pretty normal for people to navigate by GPS, not by map or even by written instructions. It’s convenient to rely on devices. But we have driven to places and had NO idea how we got there except that “Mr. Carson” – our pet name for our British Accented “voice” – told us where to turn. In other words, we accomplished the goal of getting somewhere, but not really learning the geography, or even getting a good sense of direction. And we trust our GPS at a potential cost: the instructions could be WRONG, or dangerous, and our device might die. True story 2: we accidentally wiped our handheld GPS track when our goal was to return through a heavily fogged in trail at night – depriving us of the very bit of information that we needed!  We lived, obviously, but took several wrong turns as a result.

### First we like the Planisphere because it is indeed a Map.

You can study the Planisphere day or night and observe what constellations are near other constellations.  A planisphere is in fact a rotating map. Unlike directions to grandma’s house, the appearance of the night sky changes minute by minute and season by season because of the earth’s rotation and the earths path around the sun.  While you know you can always turn left to get to grandma’s house, what you want to find in the night sky may in fact be “upside down” from what you remember 3 months or six hours ago.

From a larger map like a Planisphere you’ll discover that lining up Rigel to Betelgeuse (in Orion) and keeping straight will get you to Castor and Pollux in Gemini.  Following Orion’s “belt stars” toward the Rigel side will get you to Taurus and from there if you keeping going you’ll find the Pleiades… and so on. You’ll learn that you can navigate to the stars WITH the stars.

### A Planisphere is also a Chart of Dates

A Planisphere also has a very powerful do-it-once approach to aligning things in the night sky. Spin the wheel to the sky configuration you wish and you can read around the edges every time of night over about 5 months in which the sky will appear in the same configuration!  No app we’ve seen does that!  In fact, we use the Planisphere to decide when the Milky Way will appear over our favorite waterfall or when Andromeda will be high in the night sky so we can snap it’s picture with the minimum amount of atmospheric distortion. The planisphere doesn’t tell us about the moon, but it does give us all the dates we have to work with.

### Planispheres are Hard to Misconfigure

An app must have the correct location and timezone – which you may have noticed in True Story 1 can easily be quite wrong, a Planisphere is based on your local time. The only parameter you have to get right is to match your latitude with the proper Planisphere chart. If you live in San Francisco, you’ll want a chart that is valid from 30-40 degrees, not one that is 40-50 degrees and thus more suitable to Seattle residents. The most often made mistake on a Planisphere is to not subtract an hour from the time shown on the chart during daylight savings time. Some charts have the daylight savings time equivalent printed on them, but if not, just remember that during the summer if the watch reads 9 PM, you dial the chart to 8 PM.  The universe does not suddenly lurch 15 degrees when we decide to artificially set the time ahead an hour!

The one unfortunate thing about planispheres is that not all are created equal. We prefer DH Chandler’s LARGE charts because they are double sided and have less distortion than the single-sided charts. While it might be counter-intuitive to create a chart of black dots on a white background to represent the stars, it’s actually easier to read at night with a red flashlight than a chart with white stars on a black background.  You can get DH Chandler’s charts from Amazon for about \$13 and from many other retailers.  If you join us for any of our events, we always have a supply on hand for our students.

# Hunting Comets and other faint objects in not-dark skies

It turns out the much hyped PanSTARRS C/2011L4 Comet is not living up to the hype. Unfortunately failure to meet the over exhuberant expectations  is common since predicting brightness and visibility of an object like a comet is a difficult science. In fact, it’s part science, part black art and part good guessing – mostly the latter.

The photo above was taken on March 12 when the moon and PanSTARRS nestled closely together. The close quarters made finding the comet much easier despite the bands of clouds passing by.  The strategy for finding the comet in that case was simple: use a telephoto lens, put the moon at the right edge of the photo and take different exposures periodically and at different settings (e.g. +2, 0, and -2 stops). Then hunt for smudges.

The IDEAL telephoto lens would be one that was a few angular degrees wider than the difference between the moon’s position and the comet’s position. How to determine the position of each is discussed in the last section below. Figuring out the angular view of your lens is easy using online tools like this one from Tawbaware, makers of Image Stacker (like that program!). If you know the field of view at your minimum and maximum zoom, you can use that information to your advantage.

## Finding the Comet with a Nearby Moon

The point at the moon strategy made finding the comet easy because:

1. There is no way you’d be able to see the comet if you were not able to find the much brighter moon nearby.
2. On that one night, the comet and the moon were within 4 degrees of one another.  That’s quite close.

I know some people tried to find the comet using wide angle lenses. That strategy might work, but the comet is such a tiny thing and it’s visibility is so tenuous based on the atmosphere, light pollution, and sky brightness that you may only realize – as many did – that you captured the comet after carefully inspecting your photos at home.

The truth is you are unlikely to see PanSTARRS by eye or in your camera’s view finder unless your conditions are nearly ideal.  Hopefully ISON which is coming in December will be brighter and better.

## Finding the Comet when the Moon is Farther Away

The following night, both the comet and the moon had moved relative to the sky. On March 13, the moon was 12.5 degrees above the comet and about 4 degrees farther west (again, how I knew this is coming in just a minute).  So one simple strategy for finding the comet would be to zoom your telephoto lens so that it has a field of view of about 14 to 15 degrees in the long direction which for me, is 80 millimeters focal length on a 1.6 crop factor camera.

On a tripod with the camera in portrait orientation adjust the view so that the the moon is in the upper left of the frame. Shoot bracketed shots. Check the lower right corner of each one for the tell-tale comet smudge.  Keep readjusting the view so the moon remains in the upper left for each shot. Zoom out a little bit too, in case your geometry is a little off. Eventually as it gets dark enough or the sky clear enough you should find it.

In fact the way I found the comet last night without using my camera but by using my telescope. The program Clinometer (on my iPhone) measures angles. I sighted the moon with my 8″ Dobsonian telescope and measured the angle along the telescope barrel using the inclinometer program. I then lowered the altitude (elevation angle) of the telescope by 12 degrees to match the altitude of the comet. Then I slowly rotated the telescope northward until I found the comet.  It wasn’t easy from my urban location, but it wasn’t impossible either.  By the time I was able to find the comet it was only about 6 degrees high in the sky – that’s way too low if you have trees, hills, and houses nearby to deal with.  In theory, this strategy would work with a telephoto lens or with binoculars, however, binoculars need to be steady and where I spied from last night had streetlights in the distance and the flare and glare from those streetlights made finding the faint comet nigh impossible.

## What if there is no Moon to Find the Comet With?

Unfortunately starting on March 14th, the moon will be quite far from the comet, so the opposite strategy is required:  Use a landmark in a known direction as the starting point and look “upward” from the horizon.  In other words, zoom your telephoto lens so that the field of view covers the angle from the horizon to the comets altitude (angle) above the horizon.  Don’t forget that as the earth spins this angle changes every minute! Orient you camera in landscape mode and point it as close as you can to the correct direction (azimuth). Look along the top of the frame to see if you’ve captured the comet.

SpyGlass’s view shows the direction the camera is facing (Azimuth) and the elevation angle (Altitude)

But what direction should you point your lens or telescope? Use a compass application or actual compass. BEWARE however as the compass applications have lots of gotchas and are only accurate to about 5-10 degrees.  And if you aren’t sure how to use a real compass your local magnetic declination might bite you. Better would be a GPS with a built-in calibrate-able compass.  And perhaps even better still would be to use an application like TPE (which I discuss in my Catching the Moon Webinars) to calculate the correct azimuth from the location you plan to stand.  An application that might help a lot is “SpyGlass
however don’t forget that I found the directional accuracy of my iPhone and iPad to be pretty poor.  Being off by 5 degrees may mean looking in the wrong place.

## How Do I Know the Altitude and Azimuth for the Comet?

Unfortunately, that’s a tough one.  I use the free program Stellarium. I then added the comet to the “Solar System Data Base” (search around on the web and you’ll find instructions). I selected my viewing location, dialed in the time, did a search for good ‘ol C/2011 L4 and let it tell me the azimuth and altitude.

Above I’ve dialed up the time and clicked the moon. The highlighted line shows me the azimuth (direction) and altitude (angle above the horizon) for the moon which at that time are 264 degrees or just a little south of west, and 30.5 degrees high.  Clicking on the comet shows 272 degrees – a tiny bit north of west and 9.5 degrees.  So now we know that the comet will be 8 degrees north and 21 degrees south of the moon – and that won’t change significantly for the rest of the night.

Since we also know the direction for the comet is about due west at this time, we can apply the telephoto-lens horizon trick I described earlier.

Another way you can find the azimuth and altitude is by checking my animation HERE – note that the animation is correct for San Francisco  (and most places nearby).  There is also a table of the azimuth and elevation in the text of the Flickr post.

By the way, one way to find the right spot on the horizon is to use the sunset location as a guide.