  Cosmology in general, and relation between redshift and cosmic epoch in
particular, is usually obscure to first years university students, secondary
students, as well as journalists, politicians and the general public scientists
may have interactions with. I identify the need for a simple artifact
scientists may give to the public to clarify a few relations between redshift
and other physical quantities, more meaningful for a non-scientist audience.
This simple bookmark aims at completing previous "pen-and-pencil cosmological
calculator" nomograms. I created a small, handy, duplicable bookmark with two
printed sides, showing the corresponding cosmological values of redshift, age,
time, and angular scale (for 1 kpc), using the Planck 2018 cosmology. On the
recto, the redshift range of [0.1, 1000] approaches the recombination with a
logarithmic scale. On the verso, the redshift range is chosen to be [0, 30]
using a linear scale, covering the range of current (and future) detections of
galaxies. A few examples are given, illustrating e.g. Planck, JWST or Euclid
capabilities and complementarities, time interval non-linearity, properties of
galaxies and clusters. This handy bookmark may be printed cheaply and offered
to every student in physics (undergrad and grad student) in our universities or
to secondary schools students we visit. The Cosmology Ruler Bookmark included
is ready to print (single- or double-sided). The python script is available on
github, allowing changes adapted to everyone's needs for teaching and outreach
purposes, including with other cosmologies or applied to other scientific
fields.
