The number of page goes down,down and down

Question

I have \pagestyle{fancy} with usual settings. And names of my chapters sometimes are so huge (above the top line) that somehow the number of page (which is in foot of the page) is going down, down and down with each page until it'll disappear. It is ridiculous to see at the first page approximately 2cm between number and end of the page and 1mm on the page 100 and nothing at the last page.

%document information
\documentclass[10pt,a4paper]{book}

% packages
\usepackage{cmap}
\usepackage[utf8]{inputenc}
\usepackage[T2A]{fontenc}
\usepackage[russian]{babel}
\usepackage{amsfonts,longtable} 
\usepackage{indentfirst}
\usepackage{amsmath,amsthm,amstext,amssymb,amscd} 
\usepackage{fancyhdr}                           
\usepackage{hyperref}
\usepackage[margin=2.3cm, footskip = 0.5 cm]{geometry}
\usepackage{epigraph}
\usepackage{tikz}                                   
\usepackage{cancel}
\usepackage{upgreek}
\usepackage{bm}                         
\usepackage{xcolor}
\definecolor{darkblue}{rgb}{0,0,.6}

% page settings
\frenchspacing 
\parindent=1cm
\sloppy
\fancyfoot[C]{\thepage}

% other settings
\pdfcompresslevel=9
\pdfobjcompresslevel=9

\hypersetup{
  pdfsubject         = {Matan},
  pdfstartview       = {FitH},
  pdfborder          = {0 0 0},
  bookmarksopen      = true,
  bookmarksnumbered  = true,
  bookmarksopenlevel = 2,
  colorlinks = true,     linkcolor  = darkblue,
}


%own commands

\begin{document}

\pagestyle{plain}
\frontmatter
\tableofcontents

\mainmatter
\pagestyle{fancy}

\setcounter{chapter}{0}
\foreach \n in {1,...,36}{\include{chapters/chapter\n}}

\end{document}

And don't blame me for using text about Eminem for test in future math book. :)

PS. In chapters/chapter\n there are just texts about Eminem with no commands expect \chapter{}

Answer 1

There's a clear warning about headheight being to small (see the .log file.) It's recommended, depending on the current very long chapter titles, that the headheight is set to about 35pt at least.

In my opinion, footskip is too small here, I increased its value to 1cm.

Now the page number position remains fixed.

%document information
\documentclass[10pt,a4paper]{book}

% packages
\usepackage{cmap}
\usepackage[utf8]{inputenc}
\usepackage[T2A]{fontenc}
\usepackage[russian]{babel}
\usepackage{amsfonts,longtable} 
\usepackage{indentfirst}
\usepackage{amsmath,amsthm,amstext,amssymb,amscd} 
\usepackage{fancyhdr}                           
\usepackage[includeheadfoot,margin=2.3cm, footskip = 1 cm,headheight=36pt]{geometry}
\usepackage{epigraph}
\usepackage{tikz}                                   
\usepackage{cancel}
\usepackage{upgreek}
\usepackage{bm}                         
\usepackage{xcolor}
\definecolor{darkblue}{rgb}{0,0,.6}

% page settings
\frenchspacing 
\parindent=1cm
\sloppy
\fancyfoot[C]{\thepage}

% front page
\author{Андрей Диденко}
\title{Подготовка к ГОСу по МатАнализу
\date{16 Апреля 2016}.
\LaTeX}
\usepackage{hyperref}    
% other settings
\pdfcompresslevel=9
\pdfobjcompresslevel=9

\hypersetup{
  pdftitle           = {GOS_MatAn},
  pdfauthor          = {Didenko Andre},
  pdfsubject         = {Matan},
  pdfstartview       = {FitH},
  pdfborder          = {0 0 0},
  bookmarksopen      = true,
  bookmarksnumbered  = true,
  bookmarksopenlevel = 2,
  colorlinks = true,     linkcolor  = darkblue,
}


%own commands

\usepackage{blindtext}

\begin{document}

\pagestyle{plain}
\frontmatter
\maketitle
\epigraph{Дорогой моей и любимой Настеньке посвящается}{Диденко А.А.}
\tableofcontents

\mainmatter
\pagestyle{fancy}

\setcounter{chapter}{0}
\foreach \n in {1,...,36}{\chapter{A long long long long long long long long long long long long long long long long long long long long chapter title which is totally boring}\blindtext[10]}

\end{document}