Sky & Telescope - February 2021 - 72

Finding North by Day
Daylight polar alignment can be simple if you're prepared.

IF YOU'VE EVER TRIED to polar align
a telescope during the day, you quickly
learn how inaccurate a magnetic compass can be. Even if you correct for the
declination in your area, there's no
correcting for the rebar in your driveway,
the magnetic fields in your telescope, etc.
You're lucky to get within a couple of
degrees of celestial north.
Florida astrophotographer Alberto
Sagüés has come up with a better way.
He placed an angle bracket in a fi xed,
convenient location near where he sets
up his telescope, aligned that with true
north, then uses an angle-measuring
smartphone app (AngleFinder for iOS,
but there are plenty of Android apps,
too) to transfer that heading to his
phone. The app uses the phone's internal accelerometer and gyroscope rather
than the magnetic compass to hold its
reference position, and that's accurate
to within half a degree.
When Alberto aligns his telescope,
he first levels the mount, then he places

his phone against the angle bracket and
resets the angle-finding app to zero.
He then moves the phone to an angle
bracket on his telescope that's aligned
precisely with the scope's optical axis.
He adjusts the telescope in azimuth
until the phone app again reads 0°, and
he's aligned. With that simple alignment, he can invariably slew to see
Venus in the finder on the first try.
But how to find true north to align
your angle bracket? Several ways exist:
1: Do it at night, with Polaris visible.
This is surprisingly difficult (especially in the Southern Hemisphere!).
But even in the north, unless you live
near the equator, Polaris is quite a ways
up in the sky. It's not precisely north
anyway, except for twice a day when
it's directly above or below the celestial
pole. You either have to wait for one of
those moments or calculate an offset.
Then hang a plumb bob over your work
surface and sight past the string, moving your head left and right until the

string is exactly north of your eye. Mark
your line of sight on the work surface,
and you've got north.
2: Survey a north line using a good
map and known landmarks. Google
Earth is pretty precise. If you can zoom
in on your observing site and on a
distant landmark that's directly north
of you (or at an angle you can measure),
you can find north that way.
3: My personal favorite way (because
I'm a sundial nut) is to use the Sun.
At solar noon, the shadow of a vertical stick or a plumb-bob string will fall
directly north-south. The trick is to
determine when it's exactly solar noon.
The Sun moves 15° per hour in the sky,
so you don't want to be off by more
than a minute or so if you want an
accurate north line.
You might think you just need to
add an hour for daylight saving time.
Nuh-uh. It's more complicated fun
than that! You also need to add a correction for how deep you are into your
time zone. Time zones start on even
15° longitude lines. In the U.S., those
lines are 75° (Eastern), 90° (Central),
105° (Mountain), 120° (Pacific), 135°
(Alaska), and 150° (Hawai'i). So if you
live in, say, Altoona, Pennsylvania, at
longitude 78.4°, you'd be 3.4° deep into
your time zone. If a time zone is 15°
wide and it takes the Sun an hour to

S Left: The bracket aligned to true north is fixed near Alberto Sagüés's observing spot, where he places his smartphone to record the direction to
within half a degree. Right: He then transfers the reading to the telescope via an angle-measuring smartphone app.

72

FEBRUARY 2 021 * SK Y & TELESCOPE

A LBERTO SAGÜÉS (2)

ASTRONOMER'S WORKBENCH by Jerry Oltion



Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021

Contents
Sky & Telescope - February 2021 - Cover1
Sky & Telescope - February 2021 - Cover2
Sky & Telescope - February 2021 - 1
Sky & Telescope - February 2021 - Contents
Sky & Telescope - February 2021 - 3
Sky & Telescope - February 2021 - 4
Sky & Telescope - February 2021 - 5
Sky & Telescope - February 2021 - 6
Sky & Telescope - February 2021 - 7
Sky & Telescope - February 2021 - 8
Sky & Telescope - February 2021 - 9
Sky & Telescope - February 2021 - 10
Sky & Telescope - February 2021 - 11
Sky & Telescope - February 2021 - 12
Sky & Telescope - February 2021 - 13
Sky & Telescope - February 2021 - 14
Sky & Telescope - February 2021 - 15
Sky & Telescope - February 2021 - 16
Sky & Telescope - February 2021 - 17
Sky & Telescope - February 2021 - 18
Sky & Telescope - February 2021 - 19
Sky & Telescope - February 2021 - 20
Sky & Telescope - February 2021 - 21
Sky & Telescope - February 2021 - 22
Sky & Telescope - February 2021 - 23
Sky & Telescope - February 2021 - 24
Sky & Telescope - February 2021 - 25
Sky & Telescope - February 2021 - 26
Sky & Telescope - February 2021 - 27
Sky & Telescope - February 2021 - 28
Sky & Telescope - February 2021 - 29
Sky & Telescope - February 2021 - 30
Sky & Telescope - February 2021 - 31
Sky & Telescope - February 2021 - 32
Sky & Telescope - February 2021 - 33
Sky & Telescope - February 2021 - 34
Sky & Telescope - February 2021 - 35
Sky & Telescope - February 2021 - 36
Sky & Telescope - February 2021 - 37
Sky & Telescope - February 2021 - 38
Sky & Telescope - February 2021 - 39
Sky & Telescope - February 2021 - 40
Sky & Telescope - February 2021 - 41
Sky & Telescope - February 2021 - 42
Sky & Telescope - February 2021 - 43
Sky & Telescope - February 2021 - 44
Sky & Telescope - February 2021 - 45
Sky & Telescope - February 2021 - 46
Sky & Telescope - February 2021 - 47
Sky & Telescope - February 2021 - 48
Sky & Telescope - February 2021 - 49
Sky & Telescope - February 2021 - 50
Sky & Telescope - February 2021 - 51
Sky & Telescope - February 2021 - 52
Sky & Telescope - February 2021 - 53
Sky & Telescope - February 2021 - 54
Sky & Telescope - February 2021 - 55
Sky & Telescope - February 2021 - 56
Sky & Telescope - February 2021 - 57
Sky & Telescope - February 2021 - 58
Sky & Telescope - February 2021 - 59
Sky & Telescope - February 2021 - 60
Sky & Telescope - February 2021 - 61
Sky & Telescope - February 2021 - 62
Sky & Telescope - February 2021 - 63
Sky & Telescope - February 2021 - 64
Sky & Telescope - February 2021 - 65
Sky & Telescope - February 2021 - 66
Sky & Telescope - February 2021 - 67
Sky & Telescope - February 2021 - 68
Sky & Telescope - February 2021 - 69
Sky & Telescope - February 2021 - 70
Sky & Telescope - February 2021 - 71
Sky & Telescope - February 2021 - 72
Sky & Telescope - February 2021 - 73
Sky & Telescope - February 2021 - 74
Sky & Telescope - February 2021 - 75
Sky & Telescope - February 2021 - 76
Sky & Telescope - February 2021 - 77
Sky & Telescope - February 2021 - 78
Sky & Telescope - February 2021 - 79
Sky & Telescope - February 2021 - 80
Sky & Telescope - February 2021 - 81
Sky & Telescope - February 2021 - 82
Sky & Telescope - February 2021 - 83
Sky & Telescope - February 2021 - 84
Sky & Telescope - February 2021 - Cover3
Sky & Telescope - February 2021 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com