How to make equation number not lag behind long equation?

Question

I have the following equation. It compiles like this:

What is an aesthetic solution here to make this look better?

\begin{equation}
    {\bar{t} \over c} = {0.3 \over 0.78}\left[{1 \over 0.78 \cos{\Lambda_{1/4}}} - 0.78\cos{\Lambda_{1/4}}\right]^{1/3}\left[{1 - \left[{5 + (0.78 \cos{\Lambda_{1/4}})^2 \over 5 + (1.15 - {0.5741 \over 4(\cos{\Lambda_{1/4}})^2})^2}\right]^{3.5}}\right]^{2/3}
    \label{espesor_relativo_flecha}
\end{equation}

Answer 1

If the text width is the standard one, this works:

\documentclass{article}
\usepackage{newtxtext,newtxmath}
\usepackage{amsmath}

\usepackage{lipsum}

\counterwithin{equation}{section}

\begin{document}

\setcounter{section}{5}
\setcounter{equation}{7}

\lipsum[3]
\begin{equation}
\hspace{0pt}
\frac{\bar{t}}{c} =
\frac{0.3}{0.78}\biggl[\frac{1}{0.78 \cos\Lambda_{1/4}} - 0.78\cos\Lambda_{1/4}\biggr]^{1/3}
\Biggl[1 - \Biggl[
  \frac{5 + (0.78 \cos\Lambda_{1/4})^2}
       {5 + \bigl(1.15 - \frac{0.5741}{4\cos^2\Lambda_{1/4}}\bigr)^2}\Biggr]^{3.5}
\Biggr]^{2/3}
\hspace{10000pt minus 1fill}
\label{espesor_relativo_flecha}
\end{equation}
\lipsum[3]

\end{document}

Alternatively, use multline:

\documentclass{article}
\usepackage{newtxtext,newtxmath}
\usepackage{amsmath}

\usepackage{lipsum}

\counterwithin{equation}{section}

\begin{document}

\setcounter{section}{5}
\setcounter{equation}{7}

\lipsum[3]
\begin{multline}
\qquad
\frac{\bar{t}}{c} =
\frac{0.3}{0.78}\biggl[\frac{1}{0.78 \cos\Lambda_{1/4}} - 0.78\cos\Lambda_{1/4}\biggr]^{1/3}
\\
\cdot\Biggl[1 - \Biggl[
  \frac{5 + (0.78 \cos\Lambda_{1/4})^2}
       {5 + \bigl(1.15 - \frac{0.5741}{4\cos^2\Lambda_{1/4}}\bigr)^2}\Biggr]^{3.5}
\Biggr]^{2/3}
\qquad
\label{espesor_relativo_flecha}
\end{multline}
\lipsum[3]

\end{document}

Answer 2

I'd like to suggest that you create an abbreviation/shorthand, say \theta, for the repeated and bulky \cos\Lambda_{1/4} term.

\documentclass{article}
\begin{document}

\noindent
Put $\theta\equiv\cos\Lambda_{1/4}$. Then
\begin{equation}\label{espesor_relativo_flecha}
\frac{\bar{t}}{c} = 
\frac{0.3}{0.78}
\biggl[\frac{1}{0.78\theta} - 0.78\theta\biggr]^{\!1/3}\,
\biggl[1 - \biggl[
\frac{5 + (0.78 \theta)^2}{5 + (1.15 - 0.5741/(4\theta^2))^2}
\biggr]^{\!3.5} \, \biggr]^{\!2/3}
\end{equation}

\end{document}